If you're wanting to race, you'd probably want to look at electrics, unless you go 1/8 buggy/truggy. Check your local tracks to see what classes they run, chances are, they won't have a nitro SCT class though.
What are you looking to do with it? Both can be converted into something else later on, the Slayer can be changed into a Revo, and the Losi can be made into a buggy (810) or truggy (Ten-T).