My 2c...

I always chose the SWU upgrade over the complimentary for the following rationale:

1. As noted above, usually A inventory is more abundant than X. For most flights I take, they release one or two less seats in X than A and so a common scenario after the first round of complimentary/500-mile upgrades clear is something like F3, A1, X0 until about t-5 hours.

2. Also as noted about, A inventory can be made available before t-100 hours.

3. Importantly, a SWU clears in A and the ticket is reissued. You are now ticketed in first class. In the case of IRROPS, you will have priority in first class for a new flight. If you have X inventory, you are ticketed in Y and so during IRROPS you will be put back in Y and then can join the upgrade list below other EXPs already on it.