18" Mach 5 Sub - Check my graphs please?

Hello all,

A quick word to say that although I've only just signed up to be a member, I've trawled these forums for information in the past and can't say how helpful it's been so far!

As for myself, I'm a first-time builder looking to put together a decent subwoofer. My goal is for a speaker that will go loud and low, yet not boomy. Size is not the biggest concern provided I can get it through my door at uni! I've already given it some thought but I've decided it's time to ask someone else to check that I've understood the basics.

After throwing loads of drivers into WinISD I fell for the Mach 5 IXL 18.2.2 driver as it seemed to be able to go lower than anything else I found in the price range. My being based in the UK, however, makes this a little awkward. I've recently also discovered the Mach 5 Pi-18 which I can get direct through the iST website for ~£370.
I've been notified by iST that there is an upgraded version of the IXL 18.2.2 coming soon.

I have a few questions to the members of this board. The first few concern WinISD and what I should be looking for. I was comparing drivers before purely based on what I saw in the transfer function plots, although would I be right in saying that for comparing different drivers (with different power ratings) the SPL plot is a better comparison?

I've produced a few plots here and I hoped that someone could cast their eye over them and let me know if I'm at least along the right lines of choosing an enclosure size etc.

Green = Mach 5 Pi-18
Orange = Mach 5 Pi-18
Blue = Mach 5 IXL 18.2.2
Yellow = Mach 5 IXL 18.2.2

The orange plot is one suggested in a thread I read, but I can't understand why you'd choose that over the green alternative...why have it slope off when it can be kept flat for longer? These are the kind of thoughts I'd like confirmed or corrected!

Finally, although like these drivers I'm not dead set on anything right now. I want to do this right, and I also want to buy a good driver (in the £300 region) so that I don't simply replace it in the future, but work it in to future builds. Any of your info would be much appreciated.

Cheers
Sam