Have a britax si (not sure about the duo plus etc) which we have with isofix. None of that stupid fiddling with belts. Doesn't recline as far as non-isofix though so bear that in mind. It does recline a bit! I feel sure it has to be safer, after all, it's fixed with a bit of metal rather than a belt which moves and may be incorrectly fitted. A lot easier to fix and don't have to worry that someone has undone the belt accidentally (problem with grandma's).
I can recommend kiddicare or halfords (when they have a sale on) although halfords couldn't fit our stage 3 (another britax) properly even though they say they offer a fitting service.