Easy: buy whatever the cheapest laptop Apple sells, at any point in time. The 2ghz white Macbook is standard issue at our company; we use 'em like a chef uses a Global chef knife: software development, C code compilation, running Windows tools under VMWare, and running custom VM's for target software during analysis.
There's no feature of the Macbook Pro anyone on my team ever misses. The larger screen is a liability when traveling; at the office, you want a monitor anyways. You're almost never going to be CPU bound, and even if you were, the difference in processor performance is negligible.
Just get the cheapest Macbook and spend the rest of your money on more important things.
One caveat to this is that Apple frequently puts something at or near bottom of a range, particularly a new range, that is especially crippled. The first instance of this (Mac LC) is old enough to vote. Check out http://lowendmac.com/roadapples/worst.html to get a feel for it.
Things to look for in the order I personally prefer: maximum RAM capacity (prefer high), processor cache size (prefer large, generally), processor multiplier (prefer low), RAM speed (prefer high), processor clock speed (prefer high)
If you don't want to pour over Intel CPU specs, I'd just pick something in the 35th percentile of price and performance. Don't worry about amount of RAM pre-installed or any parameters of the hard drive. If the computer begins to run slow, pay a buddy to find a deal on Mac compatible ram and buy the largest amount your system will use. Keep regular backups. When the hard drive dies pay the same buddy to research the largest and fastest disk that uses less than or equal to the number of watts your current disk does (thermal issues make higher power/heat drives risky).
This is slightly simplified but should give you a system that has the most upgrade potential to protect your investment.
The Macbook and the Macbook Pro share the same microarchitecture. They have the same memory limits. It's hard to make a case between the two on "expandability".
The cheap one does not have VT support on the CPU which means that virtual machines run much slower than on the CPUs that do have VT where they run at native or near native speeds (depending on the VM software you use).
This becomes particularly important if you want to run a virtual Windows under heavy load or run many virtual machines.
Speaking as someone who has actually worked on a hypervisor on VT-x on a 1st-gen MB, I call totally unforeseeable bullshit on you. Sucks to be you! ;)
In fact, every Intel Mac runs at least Intel Core, and VT-x very slightly predates Core (you can get P4 parts that have it). It is in fact slightly easier to run a hypervisor on the original Core, and thus the first-gen Macbook, because the Core 2 Duo runs userland in 64 bit mode and the kernel in emulated 32 bit mode, and that's a gross oversimplification made as a small mercy.
In any case, every Macbook sold today --- even the cheapest one --- runs at least Core 2 Duo, which includes not only VT-x but also the advanced I/O stuff Intel did to make TLB flushes not kill performance.
Long story short: you can't buy a Macbook that doesn't do just fine with Parallels or VMWare Fusion.
If you are buying a new macbook today, you are right, it does not matter, they will all be fine.
I was thinking about a 13" white macbook my colleague used to use which I thought had a processor that did not have VT support (as not all Intels do http://www.intel.com/products/processor_number/chart/coreduo...) as there was an issue about it. But, frankly, it sounds like you are probably right and I have made a mistake or am confused (I won't be 100% sure until I can ask him on Monday, and Google is not doing much to help me other than to show how fast HN is being cached - http://www.google.co.uk/search?hl=en&safe=off&sa=X&#... returns this page as the top result minutes after these comments!).
I'm pretty sure I'm right: I had to hunt for an original white Macbook in 2007 because the Core 2 in the new Macbooks broke Dino's Vitriol VT-x rootkit code.
CPU's a big help if you're compiling a lot. Then again most projects are small enough where it doesn't matter. But my MBP's a necessity when it comes to compiling Firefox.
Even compilation is mostly IO bound. I'd be interested in an off-the-shelf benchmark on compiling Firefox, MB vs. MBP. I haven't seen one. Regardless, you're paying a lot for what may be a nominal difference; you could spend that money better.
There's no feature of the Macbook Pro anyone on my team ever misses. The larger screen is a liability when traveling; at the office, you want a monitor anyways. You're almost never going to be CPU bound, and even if you were, the difference in processor performance is negligible.
Just get the cheapest Macbook and spend the rest of your money on more important things.