I live in Wroughton Road and had a similar dilemma (my daughter is starting in September though, so is a year older).
I don't think Honeywell do guarantee their nursery children a reception place, in fact I'm almost certain they don't. Though you're right that they are a foundation school and have a little more flexibility in some areas.
The main differences I found :
Honeywell is smaller
Honeywell is non-uniform
Belleville appear to have more 'specialist' teachers in some areas, which is nice
Some people see Belleville as more structured, but I think the uniform has more bearing on that view than any actual teaching
I like the outside space in Belleville's reception classes (Belleville Rd site)
The expansion of Belleville put me off
The social scene at Belleville I liked
Honeywell is two separate schools - infants and juniors, with two separate headteachers
Honeywell is a foundation school, meaning they are that small bit removed from the local government (though with these new academies that could apply to many more schools) - in particular, for Honeywell it means :
They can't be told to expand their intake (so stayed at 3 form, not 4)
They 'own' the building (I think ??)
They have their own caterers as opposed to the local council caterers
The parents raise a lot of funds for new resources (as I'm sure they do at Belleville too) - the new ICT room at Honeywell was very impressive and all raised by the PTFA / parents.
The destination schools from Honeywell I felt were more suited to where we hope to be headed (i.e. more went on to independent schools) and the school seemed well tailored for this and supported parents well through the Common Entrance (some state schools we'd seen had been less enthusiastic)
We chose Honeywell. I'm still having my wobbles as we made the choice to go from a place at an independent to state, and I did like both Belleville and Honeywell - but it came down to 'that feeling' when we looked around both. Luckily my partner and I both agreed that it should be Honeywell over Belleville, but for some couples it's a tough call (though a pleasant situation to be in, obviously to be able to choose).