In general terms the earlier Spit marks are lighter and so turn better (lower wing loading). The later marks have more power and so have better climb, acceleration and speed.
The Spit Mk I has 8 .303 inch machine guns which are only marginally effective at close range and useless at long range. It has an early single speed Merlin engine which cuts out when you push the stick forward (negative Gs). It has poor aileron response at high speeds. It is outclassed in the late war arena but can give a nasty surprise to anyone flying a later mark of Spit that tries to outturn it.
The Mark Vb Spit has a two speed supercharged Merlin which gives more power. It can outturn any fighter except a Hurricane, a Spit Mk I or a Zero. It is still slower than just about anything else you will meet in the arena though. The Vb like all the later Spits has better aileron response than the I and no neg G cut out problems. The two Hispano cannons are probably the best guns in the game, powerful and accurate. They have a limited supply of ammo though.
The Seafire is basically a Mk Vb with a hook. Its a little bit heavier, so not quite as good.
The Mark IX Spit has a two stage supercharger which gives more power, particularly at altitude. It is a fair bit faster and climbs better than the Vb, but turns a bit worse.
The Mark VIII carries more fuel than the IX and has a more powerful Merlin. This is the Spit that causes the most problems for my F6F.
The Mark XIV uses a more powerful, but much heavier Griffon engine rather than the Merlin. This Spit performs very well at very high altitudes. It has tricky low speed handling so is best used as a boom and zoom machine.
The Mark XVI is essentially a 1944 Mark IX. Unlike our 1942 era Mark IX it has a Merlin tuned for more power at low altitudes. It also used higher octane fuel which allowed it to run a lot more boost than the Mk IX. This makes it perform much better in terms of speed, acceleration and climb, particurlarly at the altitudes that arena fights tend to take place at. The XVI also features clipped wingtips giving better roll response at the expense of some turning ability. Two handy 0.5 inch guns replace the fairly useless 303 inch guns as secondary armanent.
