I'm an electric guy, but for all-day outdoor, primarily off-road running, nitro is the way to go unless noise is an issue. The Traxxas Slash(assuming that's the nitro version) is a good idea, given support and part/aftermarket, but depending on the surfaces you might be driving on, a truggy might be even better.
I have any outdoor RC run nitro, indoor ones electric for pretty glaringly obvious reasons. Hack hack, cough cough, dead dead.
Oh, and just my two cents on the Slayer.....Ignore TRX's breakin method. It's 95% of why the 2.5 and 3.3 have a reputation for failing during or shortly after break-in. TRX wants you to idle around with it so rich it barely runs, this doesn't allow the sleeve to expand properly. Instead, substitute a good heat cycling method instead. I like to set my engines where they're rich enough to not hit max RPM and belch out enough smoke to worry Greenpeace, but lean enough where they still run smoothly and don't stall when you apply throttle. Then, I drive them around to get them warmed up and keep them there, gently of course. I smoothly roll into and out of 1/4 throttle for first two tanks, 1/2 throttle for next one, up to 3/4 on the fourth, and on the fifth I'll roll into full throttle. I burn about 95% of each tank up, and between each tank I shut the engine off by plugging the exhaust or mechanically stopping the flywheel, set it to BDC, refill the tank and have a coke. When the exhaust is cold to the touch I fire it back up and do the next.
Broke in my .18 CV-RX that way, runs like a top two gallons later.
If I can drive on something like this as well as grass that would be ideal:
Obviously I'd avoid the larger rocks, but the general idea is there.
RC10GT2 is more than capable. I would advise a bit of a lift, though, and some far more aggressive treads. Perhaps if you can find them, something with a monster truck style chevron tread, but with roughly the same tire diameter. The stock tires are good for a groomed, hard packed dirt, but they might as well be racing slicks on a loose surface like that.
If you still want chevron tires but can't find any in ST size, only MT size, then you can put a smaller clutch bell or bigger spur gear on to compensate. The speed and acceleration should remain roughly the same and you won't burn the clutch out.
As for running in grass, IMO nitro is still pretty king. It puts a lot of load on the powertrain, and with electrics this can very very quickly overheat things. Batteries, motors, speed controllers, they're all quite temperature sensitive. You have to have your gearing absolutely spot on. With nitro, as long as airflow is ample and you have a good oil blend and percentage(I suggest 16-18% oil), you won't cook the engine. These engines are also amazingly durable, the only reason we have a temp considered overheating is because they can and will boil the fuel in the carb if they get too hot. There's nothing inside the engine that is temperature critical once you hit 200F.