Couple of obvious questions - whats up with Win 7 (other than having to pay for it?)
And whats up with Lion? (just the revrse scrolling?)
Win7/64 vs Win XP: Yes - win xp can be alot more audio driver friendly than win7/64, but in practice that only measurable difference say when using live - can puish CPU a tiny bit higher in XP than I can with win7, else performance near identical. On balance I prefer to be running win7 than winxp for audio work - mainly down to 64 bit support and that now, you are more likely to find better supported drivers for Win7 than WinXP unless you have really old gear and drivers.
Win7/64 vs Snow leopard perfornce and ability to push the CPU use slightly higher before dropouts is about the same as Win7 vs XP. I havnt re-tested with Lion, but so far Im not noticing any performance difference with audio. The only little annoyance with lion seems to be much longer to shut down, but no difference to boot time.
On Lion vs Leopard? kind of like the XP vs Win7 in terms of driver support, old gear etc.
XP is dead, unsupported by MS, support from app and plugin developers is fading, and only 32 bit (so max accessable memory 3GB if your lucky, but for most people only 2GB) - far from ideal with many current instruments in live, even though live is still onlyu 32 bit. Live 9 (we hope) will be 64 bit as well.
I agree on the macbook air - really nicely made machine, but its allways the headache with macs - often not enough I/O so you end up needing a hub and having to be careful about which hardware gets the direct connection (usually audio interface) and which gets the hub (all your controllers). Even with the MBP17 I have to deal with this to. That said, my old Lenovo T61P only has 3 USB ports (same as my MBP17) - any more and its expresscards/hubs etc.
Also - yes apple always tell you to never install an earlier version of an OS than that which ships with the machine.
I also get you on the computer thing, personally I found that to be more of an issue using a PC than a mac - maybe something silly like the PC always reminded me of my old day job whereas the mac didnt.
In your shoes, if you like the MBA - seriously - get used to lion - it aint that bad
Keep the whole bootcamp thing in case you really have to run windows for something that simply isnt available for OSX, else I wouoldnt install it at all on an MBA - too much HD space lost to it.
I also have to wonder if an MBP13 might be better for you than an MBA (assuming you mean MBA13?).