Heaters in AC300

[Breaker4show]

What heaters fit in an AC300 ? I am probably either ordering or picking up a heater today and was looking at the Thermal Compact 50W, the Visi-Thermal Stealth 50W ( think its to tall ), Visi-Therm Deluxe 50W, Tronic 50W, and the THEO 50W! It seems like I will end up with either the THEO or the Thermal Compact! Would like advice on which one you all think is better!

Thanks,
Nick

Also its a 5.5 gallon tank!

[Sugar Magnolia]

http://petsolutions.com/product.asp_Q_pn_E_15510721_A_Thermal+Compact+Heaters_E_

It'll fit but slightly slanted.