Diesel if you need it (towing large amounts or putting insane miles on it where the gas savings make up for the cost) or you just have a great deal come along where you'd be crazy not to buy it. Gas if you don't.
We bought a 2001 Gas (Ford) 35' Itsca Suncruiser for $15k including a full tow system (no toad though) and dome satellite unit that was in mint condition on the inside and mechanically. It has double slides, 7k Onan, Great basement AC unit that includes heat pump (no propane needed at hookups), and even some neat bells and whistles some newer and more expensive rigs didn't have like heating the water off the engine heat and furnance ducted to engine heat to heat the whole thing going down the road (not just from the from vents). It had about 75k miles on it when we got it. The graphics were faded and cracked. I repainted it myself base/clear (not recommended, ton of work, take it to Mexico). I replaced the tube TVs with flat screens.
The thing serves us just as well as the $70k used DP the people we travel with have if not better seeing we have had zero maintenance needed besides standard stuff (fluids, etc) and we have taken it from Mexico to Canada and everywhere in between including some of the biggest and longest grades in both US and Canada. We have zero problem keeping up anywhere. I think we've put about 20k on in the couple years we've had it.
The two groups of people we RV with both have DPs and while they haven't had catastophic issues, they have needed to be in shop a handful of times the last couple years and it seems like diesel repairs are a lot more expensive than gas repairs. This is a small sample, and there are probably plenty of stories that say the reverse... and gassers can be just as prone to issues, and yes a diesel typically will outlast a gasser engine. But I can tell you the cost either of them have had into maintaining their diesels over the last few years I could have put in a brand new engine into mine.
I would never buy a DP from the 90s unless you needed a DP and that was what the budget allowed. Seems like it will cost more in the long haul. We couldn't live without double slides personally, not for more than a couple days. When it is closed up even with just the two of us it can feel claustrophobic and hard to get around each other.
Always buy one from someone who was anal about their maintenance. You want to buy one from the guy who keeps stacks of old paperwork and receipts just because. The guy I bought mine would take small labels all over like when the genny hours were up for the next oil change, same with motor, had labels everywhere for weights and capacities and different schedules.
You'll also find that a low mileage one that has been collecting spiderwebs in the elements for years with 30k on it can often be way more troublesome than the regularly used and cared for unit with 80k.
Good luck! Wait for the right deal, you'll know it when you find it. Again, if you don't need diesel and you're looking for most bang for your buck gas is often the way to go for both short and long term costs. Just ask someone how much it costs to break down a diesel motor for inspection let alone replace one! For me diesel is a luxury, and if I plenty of disposable income one that I would gladly take over gas. But if money is an option and you don't need the diesel then why bother?