Hard decision - they're very, very different machines. If Pilottj sees this, he can provide you with one of his brilliant car-based analogies. To paraphrase: Spitfire = an elegant but temperamental British sportscar, P-40 = a stick-shift Jeep.
I love the Spitfire - the handling is wonderful, it has none of the extreme torque or instability you'd find in, say, the P-51. You have only to point it and it goes where you want. But it's got a very short range and a very limited cooling system, which takes management and can get you into trouble on the ground - you've got about five minutes after engine start to get it into the air or the coolant starts boiling off. That means you're best off operating it from very small airfields - long taxis are not your friend. It's also happiest at higher altitudes. The A2A version with the additional Accu-Sim module lets you play with three different propellers, which have different personalities, and two engines, one in the Mk. I and one in the Mk. II.
The P-40 is much more robust - great at low altitude and short fields. It's sort of a high-performance bush plane. Cockpit management is a handful. Manual cowl flaps (like the manual radiator in the Spit) plus an electric (not engine-driven) hydraulic pump, which means that raising or lowering the gear requires moving the gear handle, then starting the hydraulic pump, then pumping the manual hydraulic lever to make sure the gear is fully in position. You can wind up wishing you were an octopus. As you can tell, I don't feel totally comfortable in it yet. But I want to, because it's kind of an outsider airplane. Wild Bill Kelso flew a P-40. 'Nuff said?
The standard answer to "which A2A airplane?" is usually "get both," and I can't argue with it here - the Spit and the P-40 give you a fascinating look at two very different contemporary designs. But budgets being what they are... your best bet is to think about what kind of flying you like to do and what kind of airplane you like to manage, then choose accordingly. In either case, be sure to buy the additional Accu-Sim module - it brings out all the realism I've been talking about.
Hope this helps.