If you have the space, I would suggest going for a 14ft trampoline - or even a large rectangular trampoline. Generally speaking, the bigger the trampoline, the more springs it has, therefore giving a better bounce.
Rectangular and oval trampolines give you a big bounce area but maximise on space as they can fit snugly into a corner area.
TP is a good brand that has been around a long time, but their most recent designs require you to replace the entire enclosure if a part of it breaks (this cannot be done easily or cheaply, Tp went bust last year so there is always a possibility certain things will be discontinued at the drop of a hat) and often leaves you having to replace the entire trampoline.
Skyhigh is a good quality brand which has a varied range, from the basic Skyhigh, to the Skyhigh Plus and then the very highest quality trampoline the Skyhigh Xtreme (they also do ovals and rectangular trampolines).
The pads, enclosures etc. are also quite affordable to replace after a few years if your children manage to wear them down with over use - but that said, the high quality pads on the Plus and Xtreme models are very durable to begin with.
You can see a range of trampolines at:
http://www.gardengames.co.uk/acatalog/Trampolines.html
or here:
http://www.trampolinesshop.co.uk/
Hope that helps - any more questions, just ask
