The ECML south of York must be one of the most boring railway lines out there, but it has the better trains, power sockets at a higher proportion of seats, better window

eat alignment and less irritating sounds.
Plus the scenery North of York is wonderful. I don't know how you plan on coming back down, but you can get VTEC from either Inverness or Aberdeen and have a really beautiful ride down. The Aberdeen trains have both the Forth and Tay bridges, and genuinely have really nice scenery out over the North Sea. The Highland Chieftain from Inverness starts off on the dramatic Highland Mainline and, for the next few weeks, is also diverted over the Forth Bridge. South of Edinburgh, there's still really dramatic views over the North Sea and, whilst the line South of York may not be all that interesting, the line North of York has some really beautiful points as well, and you do get some really nice views of Newcastle.
WCML does have its advantages, with really beautiful scenery through the Lake District. I think the tilting is really quite fun and you feel like you're going really fast. However, as has been said, there's a chance you'll get poor window alignment and be stuck staring at a plastic wall for four-and-a-half hours.
If I were you, I'd go for the Highland Chieftain, particularly if it's being diverted over the Forth Bridge. I think it's probably one of the best journeys you can do anywhere at the moment.
If you're doing an ALR, however, I'm guessing you'll be going up to Thurso by some way, so you don't have to miss out one of the mainlines! You can catch a Pendo up and come down the East Coast, or vice versa. If so, I think come up via Glasgow on the WCML, and make your way back home down the ECML on the Chieftain* for a showstopper finish to your rover.
*Or, if the timings don't work for the Chieftain specifically, you can make the same journey with a Scotrail connection