Question: Is it generally better to have a small number of fully upgraded rooms? Or have a larger number of not-so-upgraded ones? (Provided of course, that we have Power to spare.)

Wondering if the massive costs for upgrading are worth it. For example, I had a dweller training in a level 2 Strength Center with 2.00 hours to go. Then paid 4500 to upgrade it. The remaining time just went down by about 10 minutes! This seems pretty crazy since I could have built another 3-4 rooms with this cash.

Answer: 
Just to put some numbers here: 

```
Strength Level | Base    | 1st Upgrade | 2nd Upgrade
----------------------------------------------------
1 >>> 2        | 26m     | 25m         | 24m 
2 >>> 3        | 1h 20m  | 1h 16m      | 1h 13m
3 >>> 4        | 2h 40m  | 2h 32m      | 2h 26m
4 >>> 5        | 4h 27m  | 4h 14m      | 4h 3m
5 >>> 6        | 6h 41m  | 6h 22m      | 6h 5m
6 >>> 7        | 9h 21m  | 8h 55m      | 8h 31m
7 >>> 8        | 12h 28m | 11h 53m     | 11h 21m
8 >>> 9        | 16h 2m  | 15h 17m     | 14h 36m
9 >>> 10       | 20h 3m  | 19h 6m      | 18h 15m

```

Source: <http://www.ign.com/wikis/fallout-shelter/Weight_Room>. Other rooms have very similar (though not identical) numbers.

So you should be really only updating those if you have some ungodly amount of CAPS and it's burning a hole in your vault.

### Update

I have some 700k caps now, and I really don't know what to spend them on. Perhaps, now might be the time to upgrade some training rooms. However, more than half of my dwellers have full SPECIAL now and training rooms are half-filled, at best. Again, upgrading training rooms wouldn't have any effect, other than making me 50-100k caps lighter.
Build across then in. It's much cheaper to build across 2-3 rooms than it is to upgrade. You DO want to upgrade though, it increases your storage which is your main buffer against disaster. Also by building across first you save yourself the grief of having to delete a room just to expand one you really need.