maybe it would be better to go with separate pennants so that if one part goes out it can be fixed
That is certainly an option, but IMO, it is an unneccessary expense. The MH you get will be warrantied and if one has a prob, it will more than likely occur real soon, then the manufacturer will simply replace the entire unit.
there is no fan in the aquiline busche may be a bad idea
Fans are easy to add yourself if need be, but if you don't want the trouble, there are other manufacturers that do put fans in the fixtures. I'm sure there are plenty of sites that will let you "build your own", choosing different options.
like the idea of not having light on tank so that it can be raised and lowered
I agree I like the idea as well, but to be honest, I have never raised or lowered my pendant, it is a pain to get it right again, when I add new corals I adjust the photoperiod to acclimate new animals.
For a 75g I would probably go with 2 X 250W MH I am partial to the 10K Ushio bulbs, although Hamilton 10Ks and Aqualine Bucshke are comparable. I would also add some PC or VHO actinic.
With either the hqi or mogul MH you will be able to keep clams and SPS, what type lighting you use for actinic supplamentation would really be preference. I have no experience with T5 so I can't comment, for asthetics I prefer VHO to PC.