The two processors Seem to be:
http://ark.intel.com/Product.aspx?id=50067
http://ark.intel.com/Product.aspx?id=52219
and the differences are more than just clock speed - the faster one also has faster memory bandwidth, along with plain old 'more features' such as VT-d Virtualization for directed IO, Execute disable, Quicksync video, wireless display, mywifi, 4G wimax... but what it means for those things to have built in support in the CPU, I don't know.
I'd predict that the memory bandwidth, which is about 20% higher in the faster chip, would have more of an observable effect than the 10% faster CPU, but that unless you have an intensive use planned the difference wont be worth much worrying about.
Far better to look at an SSD instead of a normal hard disk, that will provide a big shift in the feel of how responsive everything is - the hard disk is the biggest bottleneck in normal computers these days. (Where a hard disk can shift 20-40Mb/s sustained, an SSD can shift 100-200Mb/s sustained. Where a hard disk can handle 100 operations per second, an SSD can handle many hundreds or a few thousands).
For which types of applications will the different graphics cards have an observable effect in performance?
- Graphics heavy games, racing and running around shooting, flying and the like (not cards, dice, board, web/flash games, etc).
- Graphics heavy apps like architecture modelling, 3D scene rendering, Pixar style film rendering.
- Currently niche apps which make use of the graphics card as a spare processor - at the moment this means things like distributed computing project SETI@Home, and PowerDirector 7 video encoding software. However, there is a push in the industry to make this more widespread, but that's probably still too far away from every day uses to bother about for another year or three.
My vote is that unless you have a particular intensive workkload or unusual use which you haven't mentioned, the 2.0Ghz will be fine, and if you can spare the money then see if you can find a machine with a good SSD to compare, and consider one as an upgrade, for an everyday snappiness boost. (Apple supplied, or aftermarket).
Since people are still commenting on this, I will post what happened as my answer:
I took the MBP to a different store where they concluded that it was a faulty HDD cable since I had installed the SSD myself and they assumed it got some kind of shock for me not wearing an ESD bracelet. (Although as far as I can remember, the friend of mine who installed the SSD for me did indeed wear the bracelet, however I probably didn't use it when I installed RAM 1,5 years ago)
It seemed to work fine for a while but after that, the glitches started to reappear, and it became so bad that the MBP couldn't boot into OS X or Windows (stuck on grey screen with glitchy stripes).
Once again, I took it to the shop, they concluded that it was a faulty logic board, which would cost me a fortune to replace. However, thanks to EU consumer laws, I have the right of up to 3 years warranty if I could prove that it's the manufacturer's fault. So they let me fill in a form and send it in along with my receipt. By the beginning of July, Apple finally approved to replace the logic board for free. For now, the MBP is working properly again.
Best Answer
The failing GPUs in these units are all subject to failure in a timeframe most would call premature. This is because the the cooling system and the generation of and the vulnerability to heat of the AMD chips are mismatched. All official Apple repairs were nothing more than swapping out the entire logic board and replacing it with the exact same thing, spec wise. Above normal room temperature and really stressing the GPU with 3D rendering while a second monitor is plugged in (getting even more heat from the thunderbolt chips) is a surefire way to further shorten the lifetime of the AMD chips.
Investing into a used machine from 2011 with AMD chips inside is a bad idea unless it is really cheap. That is because the issues can be indeed latent in the early stages. The problem starts with different symptoms and in different stages, some of them might be gone for a while, especially if everything else is unplugged, the fan speed upped to ~2500–3000 (almost inaudible for most people over 19 or so), air conditioned room, etc. [To use a AMD chip in this generation of MBPs the chip itself has to replaced with a more robust, later, cooler revision and ideally the cooling system improved (better thermal compound better applied and additional holes for the airflow)] For an example of latent issues in the early stages see this post.
But most frustrating would be a common practice popularised on youtube and other sides: reflowing/baking the logic board. That is a failed AMD chip is heated to 120–200 degrees Celsius and thereafter shown off as a re-animated corpse. But only for a short time compared to the time it was running before with no apparent problems. This procedure has a kind of half-life until the problems come back, and worse that time, breaking even sooner than before and eventually failing completely with no amount of baking doing any good any more.
That means that a self-sourced OS is of only limited value to protect against a rogue seller. A GPU treated in this way is dying soon but likely not in the time it usually takes to make a quick test of basic function in a typical sales situation. This is a latent problem in my definition.
Nevertheless it is not really possible to protect yourself from a rebaked chip. But it is possible – in a limited way – to guard against someone showing off a GPU-switching Mac with only intel GPU active (like certain hacks):
Boot your own OS. That is: macOS from an external drive. Then go into Apple Menu>About This Mac>System Report and check what is listed there. Since your own OS will contain the drivers but not the hacks: Manipulated to iGPU-only Macs will only list the IntelHD3000 chip there. (But also very likely hang on boot with your stick/drive) If you see trouble on boot in this configuration: reset the SMC/PRAM/NVRAM, reboot again and if there are immediately visible artefacts: then confront the seller!
Boot your own OS. That is: try a LiveLinux like Ubuntu. Problems already manifest show up quicker there since even the basic acceleration there (without the proprietary drvers) stress the chip too much. Also check the hardware from your chosen Linux distribution.