I had the same choice a couple of months ago travelling from Edinburgh to Liverpool. I chose Virgin in the end but you are right about the fares being a lot cheaper on TPE. I would say for that fare, if the seat is comfortable (as NIRScot says) and you can get a tea or coffee then it is worth giving it a go. It is the same choice for me from Bath to London, FGW into Paddington or a slower but cheaper Southwest into Waterloo. Both have their plus and minus points.
NTarrant, I have used XCountry extensively and for me there is little difference between them and East Coast / Virgin except that their fares are becoming ridiculously expensive. You can see a trip report for a XCountry trip on this thread.
Speedbird, do add a trip review if you use TPE. BT have created a section for rail reviews in the forum.