I think two in line 300W heaters will work really well. Remember, they only need to heat the tank 5 to 12 degrees over room temp, depending on your room temp. If I remember the spec chart that came with them, there will be no problem whatsoever. I am satisfiied with the single in line heater I have on my 55 gal
edit: the in line heaters are a couple of inches thick, and about 10 inches tall. They need to be verticle to prevent air trapping. You will need room for them to fit behind the tank, or enough room for them to fit above the canisters. If they can not be installed directly over your canister due to the head room below the tank, make sure you leave enough space behind the tank.
If this is your first tank, I would go with the canister filters. You have enough stuff to learn and deal with already without building a sump.