what is the best 17.5t motor

[oldboy]

Hi all
I am looking at getting a new motor to go with my shiny new vampire sr1+ my wife has just bought me for my birthday, at the moment I run a 13.5tmotor on a small indoor track and was wondering whether to go to a 17.5t as this seems popular at other clubs so I could go elsewhere without having to change motors each time. my question is what do you guys rate as the best motor around £50 or below(as funds are restricted as my son also races) also 17.5t or stick with 13.5t.
Thanks in advance Bob