If it's a long journey and I can possibly make some, or all, of it on loco-hauled trains, as opposed to trains with noisy motors underneath the carriages, I'll opt for that. Also, price is a big factor for me too.
For example...
Back in 2003 I travelled from Bingley (West Yorkshire) to St Ives (Cornwall). The bulk of the journey (Leeds-St Erth) was made on a Virgin Voyager. On the positive side, it was direct. On the negative side, the route via Birmingham never really seems to get any speed up, which is frustrating, and the constant droning of the motors becomes really oppressive after a couple of hours.
In 2004 I made the same trip but this time went Leeds-Kings Cross (IC225) and then Paddington-St Erth (HST). Consistently higher speeds meant the journey felt more satisfying and took no longer, despite the transfer at London. Relatively quiet trains, on account of being powered by locos rather than under-carriage motors, were a much nicer place to be, and the whole trip only cost about half as much on account of GNER (as it were then) and FGW offering much better advance purchase deals than Virgin the year before.
For shorter journeys it's really just about speed and comfort. Leeds to York, for example, can be done in just over 20 minutes with TPE but some of the Nothern services take the best part of an hour. I'll happily wait longer at Leeds to get a TPE service, even if it ends up saving me no time, because I just find it more satisfying travelling at a higher speed when I am actually on the train. Also, TPE's trains are generally quieter than Nothern's. Even if you get a 185 it's a nicer place to be than anything Northern has to offer on that route.