Dave Butler wrote
If we install the HSI, the no-longer-used CDI hole can be used for the electric
AI that's been waaaay over on the other side of the panel.

Actually, if your primary AI is vacuum and the secondary is electric,
you can simply remove the T&B and install the second AI in its place.
AC91-75 permits the replacement of the T&B with a second AI, as long
as the power source for the 2nd AI is different from the power source
for the 1st AI. So really, being able to free up the hole should not
factor into your decision.

Some people love HSI's, some hate them, some are indifferent. I've
flown several planes with HSI's and I'm indifferent. It's OK. A DG
with CDI is also OK. About the only time an HSI really has an
advantage is when you're flying reverse course on a localizer. Other
than in training, I do not believe I have ever had to do that.

Michael