Let me guess: is your MBP from 2007/2008? I have an early 2008 MBP with your specs, and it has gotten painfully slow for working with a large Aperture photo library. I've been researching what to do for the past year. From what others have said, replacing your hard drive with an SSD will significantly speed up operations that involve reading and writing photo data (such as scrolling through a large Aperture library), but won't have any effect on operations that involve processing or RAM (such as applying filters). SSDs have finally dropped enough in price that I've been tempted to get one; for instance, a Crucial 512 GB SSD was on sale last week for $350 with rebate. I hesitated and missed the sale, though. Ultimately, it's probably not worth putting much money into a computer that old, unless a new computer isn't in your budget at the moment and you just want to spend as little money as possible to extend its life another year or so. The next version of OS X probably won't work on an early 2008 MBP, because the 2007/2008 MBP was the cut-off for Mountain Lion.
The Mac Mini is a great little computer, but I personally wouldn't consider it portable, unless you don't mind also moving around the monitor, keyboard, and mouse, or unless you have multiple setups in different locations. If you don't need portability, then an iMac will give you a lot of bang for your buck for photo editing. If you do need or want portability, then your options are a MBP with Retina, a conventional MBP, or a MBA. Then it comes down to your needs, preferences, and budget. Are you a professional who can justify spending money on the best? Get a 15" rMBP. Are you a serious amateur who wants the best and can afford it? Get a 15" rMBP. Do you frequently use a DVD drive and also want the ability to easily upgrade RAM and storage? Get a conventional MBP, which I suspect will be discontinued in a year or so. Do you want low cost, extreme portability, and a processor that isn't quad core but is still much faster than what you have now? Get a MBA. Do you really need a new computer now, or can you wait until next summer? If you can wait, all of the MBPs and MBAswill be refreshed with the Intel Haswell processor, which will provide better graphics and longer battery life. Do you usually work on an external monitor? Then skip the Retina MBP (unless you have the money) and get one of the cheaper MBPs or MBAs, along with an external monitor. Personally, I'm leaning toward waiting for an 11" Haswell MBA and a Thunderbolt display.
I don't believe you can use your current MBP's display as an external display for a Mac Mini, but someone no doubt will correct me if I'm wrong.
