I have both sorts of track, my preferance for surface and design is scalextric sport - I prefer the higher grip, less brittle plastic and higher magnetism of the rails. It isn't 'better' than Carrera, just suits me more. Carrera's insulated rails is definately a good feature of their tracks too.
Size as said above is a big factor though. If you want to race 1:24 ever, scalextric may be a little small.
The other thing to consider is the rest of the system. The scalextric banked curves will probably become more available worldwide soon - it is a new product. It is very subtle banking, it isn't Carreras wall of death though, so the style of track and banking may determine the system.
Also there are fairly fundamental differences in the digital systems available (new products - so the accesries of each may expand). Scalextric is the more compatable and easy to adapt other cars system, at the moment it SEEMS to be the best thought out of all the systems, but it is a new product, and it should be said that all the digital systems provide a lot of fun and work very well.
(Carrera digital - and i think SCX too - offers a working pit lane. This is great, and whilst Scalextric say they are going to develope one too, who knows when, or if they will go through with those plans)
Scalextric is also supposed to be bringing out 'World' system shortly, which will add internet racing, programable pace cars and telemetry of your driving plus loads of other features, so if that lights your fire, theres another incentive for that direction.
Hope I don't sound like a scalextric rep - but I do like the stuff!