I'm no fan of Pendolinos, but even they wipe the floor with the 800-series from a passenger accommodation point of view. The austere interiors of the IEP units are a real let-down, and the ride quality is fair-to-middling at best.
Have you tried a LNER one? The interior ambiance I find very similar to a Pendolino other than the lighting maybe needing knocking down a notch or two.
Otherwise:
Ride: both equally iffy though in different ways. Pendolinos very often hit the bump stops, 80x are a bit jiggly but no worse than 350s.
Seats: Pendolino wins, but the old fashioned thick backed design is one reason for the poor legroom so this is balanced a bit.
First Class: Pendolino wins by a country mile. Best First Class in the UK. I think it's a real shame the IET First Class doesn't meet this standard, and there's no particular reason it couldn't.
Interior ambiance: depends which IET. The GWR ones are beyond drab, but the ones with some colour on the seats are nice enough. And none have the "fag ash Lil" beige ceilings which are rather dated.
Legroom: IET wins far and away - this is very important to me, being tall and having had DVT so being cramped in could genuinely be life threatening by causing a recurrence
Luggage space: IET wins far and away; overheads are huge
Windows: IET wins, much bigger
Overall I'm quite glad MKC's Avanti service to Brum will be an IET soon enough (minus the bad seats), and it will make me use them more (as things stand I often choose 350s even on an interavailable ticket). I don't hate Pendolinos, but I just find IETs nicer.