Been out to play today and set up tent and bivy side by side for a quick and hopefully useful comparison. The tent is the Go Outdoors Phoxx 1, the bivy is the Outdoor Research Alpine bivy (for sale…).
Pack size is pretty much the same, although the tent is a bit bigger with the poles in the pack.
Set up is way easier with the bivy, 5 pegs, a hoop and you are ready for bed.
Tent is a bit more messing about, inner first with poles and pegs.
Height difference might be important for stealth camping and headroom.
Actual space to sleep in is probably the same, but the tent has more options for kit under the flysheet.
Tent is significantly more hassle to pack even in a light breeze, and weighs about 0.6 kg more than the bivy at 900ish grammes.
Any questions , carry on, I’ve not slept in either yet!