The old saying, "If it ain't broke, don't fix it" springs to mind here. I would keep the HH3 and separate modem. If you feel the need to change the HH3 it might be worth considering a 3rd part router rather than using BT Homehubs. There are plenty very good routers for either the same price as the HH5 or even cheaper.
I don't know if the HH5 is fundamentally faulty but have a read through the forum for various problems with it.
Mine came today, ive been on infinity nearly 3 years now, ive had about 10 resets in the past 5 hours, not good at all.
The firmware seems to be the old one, i read somebody had a new firmware recently.
Had the line replaced last Thursday-so far only one reset on Sunday....fingers crossed!