I recall that on previous occasions when the train cioouldn't be divided, either because of lack of motive power or a fault (I think a failure of the coupling at the rear of the train was an issue once) the whole train cannot go to Glasgow because, even in the shorter winter configuration, it won't fit in Glasgow Central. (Edinburgh can, of course - it handles a full-length Highlander every night). So it either goes to Edinburgh or terminates at Motherwell.
Which half is fuller is unlikely to be the determining factor. Hopping from Glasgow to Edinburgh, (or vice versa) will still get you there at a reasonable time (I've done it twice when the Edinburgh was full and the Glasgow still had space - it nicely fills the time between being turfed out of the Sleeper and things starting to open - and during the Festival season Glasgow is usually cheapoer too.
Someone suggested giving Glasgow passengers the option of taking an Avanti from Carlisle to Glasgow. The original post was made at 0738. Looking at RTT the 0756 Avanti was cancelled so the next services to Glasgow were the 0809 Scotrail via Dumfries which arrives at Central at 1035, or the 0913 Transpennine which eventualy left Carlisle half an hour late and got to Glasgow at 1113. The Sleeper got to Edinburgh at 0921.
(The 0651 Transpennine was running half an hour late so had the decision not to run the Sleeper to Glasgow been made a little earlier that might have been an option)