When talking to the Baby on the Move lady - she indicated that the capsules with bases were safer than the ones that only clipped in with a seatbelt. The reason she gave was that once the base is installed in your car properly and safely - the capsule will always be secured properly and safely in it no matter how many times you clip the capsule in and out. Whereas the seatbelt-only capsules have to be re-secured every time you put it in the car and it is easy when you are in a rush to not take the time to make sure that you have clipped it in correctly and safely.
Hopefully that makes sense?
But as heaps of people still use seat-belt only capsules - I'm sure they are still very safe.
Personally, when looking at capsules - I found the Phil & Teds capsule to be nice and light but it also looked very flimsy. We went with a Graco (with a base) instead - they are fantastic. Just my opinion though - Good luck!