I have had cheap Cateye's and cheap Sigma's. I much prefer the Sigma ones, they have a bigger clearer display and will also let you input a previous odometer reading which is useful from your old computer. I have had a Cateye Velo 8 and the cheapest wireless one that they did (I am sure it was a Velo wireless) and neither allowed you to do this (checked the instructions and online). This really anoyed me with the Velo wireless as one day it decided to lose all it's data for no reason at all. The more expensive Cateye ones allow you to do this, I have a more expensive Cateye wireless one with cadence that I got off
Ebay as it was cheap and it has been fine. Why the cheaper ones (well the ones I had) cannot is pretty poor IMHO.
I lost the Velo 8 when it decided to come off its mounting. The twist lock mounting on the Sigma BC1009 that I replaced it is a much more secure system.