I've not taken an in-depth look at either (Edit: below - down the rabbit hole we go
), however it appears that the drivetrain is significantly higher end on the 9.0 (11 speed, GRX 600/800 versus 10 speed GRX600 and Tiagra) so there's additional value there. It might be worth looking up the different component costs and seeing how much of the price difference between the bikes this makes up.
Full specs for each can be found here for anyone wishing to take a deeper look:
8.9,
9.0.
I don't like the fact that the 9.0 uses a press-fit BB and an adaptor, that used on the 8.9 appears to be threaded (better IMO) but is mentioned specifically in relation to FSA's own cranksets so I'm not sure what the upgrade path would be like if you wanted to fit a better crankset in future.
As has become commonplace Boardman have saved money on the 8.9's crankset by fitting an FSA item rather than the in-group Shimano alternative; which is a shame and makes the GRX item on the 9.0 all the more appealing. Not done the numbers but it appears that the cheaper bike has a wider range of gearing (at least more at the bottom end) thanks to the much wider range cassette at the back. The pricer bike has two teeth less on the crankset though; so on the one hand a shade less efficient on paper; on the other it would give you a much lower-geared setup if you paired it with a larger cassette on the rear; if that's something that matters to you.
Both run thru-axles and hydro brakes, which is grand, as is the fact that both wheelsets are tubless ready. The seatpost on the 9.0 is smaller diameter, which (along with the frame material) might make the ride a bit bit comfortable.
Personally I'm not a fan of either frame material and a lot of the frame's behaviour will be down to the design characteristics of each example, but of course all things being equal a lighter bike is nicer (you'll not notice it much until you have to chuck it around over uneven ground).
FWIW I think the 9.0 is a nicer looking bike.
It looks like the same old story really; the high end bike is certainly better but not to the extent where it scales linearly with the price. I had the same dilemma when I bought my Genesis; although that was a bit easier as (while still disproportionate) the price difference ended up not being as much (got a great price on the higher end bike) and there was a hydro/mechanical disc situation which made the choice easier.
Good luck making the decision - the one think I can say for certain is that I'm glad I don't have to make the choice
