Strange - at the end of the day it's all about how much to change but from what you're saying the 2 Renault garages offered considerably different trade in prices with the same replacement car purchase price!?!?
A good way to see what your trade in is worth is to look at Vauxhall
click on OWNERS and then VALUE YOUR CAR, you could also try Sell Your Used Car Online For Free - We Buy Any Car
. they will give you a price to buy your car leaving you to then go and do a deal on your new motor with no trade in.
ps... welcome to the forum