I can only comment on the 350 which is more of a conventional tunnel shape. It's spacious where the 450 is cavernous with a more domed shape.
Can't really fault the design of either as long as the zips don't break. My only concern is durabillity at this price point in the market but mine seems well constructed and finished. I recon if you look after it and don't overstrain it it should be fine.
It's been soaked and hasn't leaked.
It's remained taut and un-flappy in a medium breezes.
I like big porches so you can get out of wet clothes under cover and not get the sleeping compartment wet. The Betas are more than adequate for this and vent on 2 sides so indoor cooking's not a problem.
The windows are useful too.
I was tempted too Ed and haven't been dissapointed. Great value for money.