The new paint / carpet smell will largely be gone by the end of July. In my case, I got rid of much the smell by whacking up the a/c for half an hour, then switched it off while I was sleeping, and presumably got acclimatised to anything else. In real life I'm a sheep farmer so I have a particularly acute sense of smell. By the end of July that shouldn't be a big factor, but it will smell like a new building then.

In your list, the Hilton T4 and Sofitel are simply better quality and more comfortable hotels. Crowne Plaza is a new hotel built by the same people, I wouldn't see the logic of paying (probably) more for something quite similar product wise and having the travel hassle as well. It also smells new. The Thistle is an older hotel but I and several others have reported it to be particularly comfortable, on the "pair of old slippers" principle, and that should be cheaper. I'm not quite sure why this happens actually, the Thistle is an odd one really. There's also HGI Hatton Cross to consider. If you have young children involved then consider the Staybridge Suites, which is all suite hotel on Bath Road, so bus or taxi required. Funnily enough I stayed there shortly after it opened, and somehow it didn't have a particularly strong new building smell. It is still only a few months old.

Ultimately HGI T2 is the best connected hotel in Heathrow, full stop. It's a 10 minute walk to T3, no traffic or taxi dependency. It's not the best hotel in Heathrow by any means, that honour belongs to the Sofitel and quite a few others are ahead of HGI T2.