There are pros and cons to the choice.

R12 is not as picky on charge amount affecting performance, hence a slightly mischarged system will still cool very well. R134 needs the exact charge to cool well.

R12 has larger molecules and does not leak out as easily in the case of a slight leak. R134 will escape quickly and you will quickly feel it at the vent temperature increase.

R12 cost many multiples more than R134. Seeing as with an older car you hyave a high risk of needing repairs/recharge every couple years, the cost can add up.

R134 is much more environmentally friendly and more readily available.

On the 700/900 volvos, system design is not much of an issue to run r134. On 200s, you have a small evap and somewhat inefficient codenser which does affect performance with R134. Still, I converted my 240 to R134 due to the ease and low cost. Cools fine but so-so on vent temp. Held the charge well thoughout the winter, still seems to cool nicely. My 940 was converted to r134 and blows icicles.

I vote for the conversion simply because of the long term cost effectiveness of it. Make sure they do the job right though, get out as much of the mineral oil as possible, flush wherever possible, and put the right amounts of ester oil in there. Your system should cool well.

Good luck,
Greg Mustang
Montreal - Ottawa
Canada