I had a go in a Westfield, an SEight (which is what I want to build), at the factory. Its incredible how they just give you the keys!
It was nowhere near as good as the Dax Rush because its not evolved and is a bit behind the times.....
The Dax is a nice kit too. Didn't successfully blag a test drive there, just a ride, but a mate of mine built a 270BHP Rush of which I have intimate knowledge. I'd say the Dax is best if you're looking at V8 / Turbo levels of power. It handles big power better than the Westfield chassis and the engine bay is roomier but it's a big beast and quite draggy which, IMHO, would make it less entertaining at more modest power levels.
If I were to do it again they'd be the 2 kits I'd be considering (assuming I couldn't afford an Ultima).
Mate of mine bought his kit part-built from a guy who'd given up and it's an excellent way to get a bargain. I think he paid 3k for a rolling chassis plus bodywork, wheels, tyres, the whole drivetrain. Essentially everything he needed bar a few details. Unmolested cossie Turbo engine with 6K miles on it, ecu, T5 gearbox, etc. etc. A bit of tidying up, some paint and an SVA test. Job done. One sports bike-eating car

Kevin