Hi,
Based on
The cheaper factor speaks to me
the two servos or an L293 twin motor might be the best approach.
I have just built a simple RC Toy/Robot from these chassis parts, a standalone Arduino and a L293
http://proto-pic.co.uk/tamiya-70168-double-gearbox-kit/
http://proto-pic.co.uk/tamiya-70157-universal-plate-set-2pcs/
I am sure there are other chassis options, others might like to suggest for a budget Arduino bot.
I will put pics and code for mine up on my blog in a few days.
Duane B