2-cycle is faster. It is generally where the top drivers go – but there are some very good 4-cycle racers, and plenty of below-average guys in 2-cycle.
4-cycle may or may not be more reliable.
The new 2-cycle TAG engines (Rotax Max, Parilla Leopard, etc) and the Stock Honda or TAG-shifters are pretty reliable (good for 1-2 seasons between rebuilds), especially when racing at the club level.
Regular maintenance (non-rebuild) costs will be higher on the 2-cycle than the 4-cycle.
If going 4-cycle, the Briggs LO-206 is as inexpensive as you can get when it comes to total cost (buying + maintaining). There are more “open” 4-cycle classes in some areas where reliability and costs can go up significantly.