The 1994 940T has three improvements which I like:

1. The improved intercooler with middle inlet and exhaust ports.

2. The transmission output shaft runs in a bearing instead of a bushing.

3. The engine has piston oil cooling.

The 1991 is still a fine car. There are other differences, but those are the three improvements I like the best.
--
john

The first thing I would go by is condition and evidence of maintenance, then possibly mileage, but I would rather have a well cared for 200k car than a neglected 100k car. If you find a well cared for 100k car then it's a no-brainer. Cost is going to be a factor, obviously what you need to pay for the car but also what will it cost to make everything "right"
If everything else was equal I'd take the 94 940 because it's a little more modern and has some upgrades in equipment and mechanicals that the 91 wont have i.e.:
piston cooling oil squirters, passenger air-bag, and if it wasn't so early I could think of more.
I'm sure you'll get more responses/information soon. Asking this board which Volvo is better always opens a can of worms.
Good Luck