Which macbook reigns supreme?

IMHO, more ram will help, so I'd go 13"!

Then again, I'm typing on a 13" MBPR and it's my perfect size for a laptop. Occasionally I wish for a larger screen, but you can take care of that eventually (if needed) by getting an external monitor. I'm not sure how different the two processors will feel, but I just love the 13". You can't really go wrong, but remember: you can't really upgrade RAM on these things.

Get the 15" Retina MacBook Pro with the upgraded memory to 16GB. Memory reigns supreme.

Great question. It's something I stressed over for quite some time, myself. To be honest any MacBook Pro will most likely handle what you throw at it. My thought process when I get a new machine is buy something that will work well for a long time, which usually equates to something more top of the line. Buy better specs, and it should last longer. Let's look at the differences here.

RAM: Arguably the most important, and probably cheapest, component, this will allow you to run more programs at once without lag. If your committed to buying one of the best laptops on the market, I would upgrade to 16gb. Also, be aware that you will need to allocate machine resources to running that virtual machine. For example, if you launch a windows instance with 4gb of ram, your OS X now only has 4gb left, until you shut down the VM. You will be allocating processing resources as well.

SSD: Both here are the same, but should note that SSD dramatically improves read/write time, and you will see significant performance gains from that alone if coming from a traditional HDD.

Processor: The question I would have here is whether the i5 is a dual or quad core. The i7 most certainly is quad core and will allow more tasks to be done simultaneously. Without going into too much detail, I believe each core should have two threads, and one of the features of the i7 line is hyper-threading. There are other considerations like L3 cache, turboboost, actual processing speed, and the level of integrated graphics. Here's a brief article.

Screen Size: A smaller screen will give you a slight bump in portability, but sacrificing screen real estate can be quite constricting and a nuisance when interacting with multiple programs at once. Judging how much you will be on the go and whether you will have a monitor to plug into can help aid in this decision. (note: I have a 15" and more times than not I'm wishing for more)

Graphics: You have not mentioned anything about a discrete graphics card , probably because Apple has dropped them, save from the high end Pro 15, but it is something to think about or at least understand. The newer line of Intel chips have drastically improved integrated graphics capabilities (built into the processor), so probably not a concern.

I'm really not sure the depth of what you are looking for. If you want more specifics I could probably point you in the right directions. Either of these will perform great for your said needs, so it sounds to me like you should determine how portable you want to be, if screen size itself is a concern, and the longevity you expect.

Get the 15" with the i7 CPU, and upgrade the RAM to 16GB. i7 CPU has hyperthreading feature, which would be essentially double the number of processors. I also assume that the i7 in the macbook is a quad core, which would give you 8 threads (vs 2 threads for an i5 CPU or 4 threads for a low-end 2 cores i7 CPU model). When partitioning for a virtual machine, you might have the option to assign the number of cores to the machine (at least on my PC virtualization software).