Isofix is a convenience. Seats fitted correctly with the seatbelt are of the same safety standard than one fitted with isofix. The issue is whether you are confident you can fit the seat properly everytime.
My personal preference is that i would go for isofix for the HBB stage if possible, as then there is something holding it in to the car and I wouldn't have to keep remembering to strap it back in when Zoe has got out, but I wasn't bothered about having it for the previous. It will also depend how often you are puttig it in and out though as if I was swapping it between cars frequently then i would be inclined to go for isofix.