Well, there's a couple of mil-sim considerations for you...
Tippmann A-5 or X7: (MP5 and SCAR-L respectively).
BT Combat, BT-4, BT Delta.
+:
Highly modifiable.
Good reliability.
Offset hopper is good for adding sights (for BT series, you need to buy the ripdrive).
Can use CO2 or HPA.
-:
They're heavy.
Mods are expensive, so are the markers to begin with.
A mass of internals to take care of (and lose).
They're loud.
They're not electronic.
High recoil, low rates of fire.
Smart Parts SP-1 or SP-8.
BT TM-7.
+:
Electronic markers, different firing modes.
Relatively quiet compared to the Tippmann/BT counterparts.
(SP-1, TM-7) Loads of modifications (e.g. they accept a host of already available stocks, sights etc.)
Can use CO2 or HPA (not 100% sure if that is true for the SP-8?)
Low kick with better rate of fire,
-:
(SP-8) Still relatively heavy.
Battery requirements.
Electronics going bad are generally more expensive to fix or replace.
However, I'd like to stress the fact that ANY marker can be used in the woods, you can buy a £1000 SL-94 and it'll perform just as awesome as a £200 Tippmann in the woods, and in scenario.
Obviously as a new player I don't advise you actually go out and buy a SL-94, but anything will do... a Proto Matrix Rail is a good start, or the Smart Parts Ion XE... it really depends on if you *really* want to go mil-sim or not.