I am looking at two similar Rangers this afternoon. One has a 250 Mercury Pro XS and the other has a Yamaha 250 HPDI.
I am leaning to the Mercury because of things I have heard (mainly forum talk) about the HPDI's.
In as civil a way as possible can I get some opinions on each to help me make my decision?
Also, what are some good questions to ask the owners tonight when I'm looking?

You might also consider speaking with a couple of outboard mechanics and get their unbiased input on both engines.

I would feel safer with the Merc. Those HPDI's are ticking time bombs when it comes to popping the powerhead. If the HPDI powerhead has been replaced with a series 2 powerhead I would feel much better about the Yammer hammer.

When I was looking at boats......after reading and talking with several people, I COMPLETELY AVOIDED the HPDI.

The gas mileage on the Optimax will out perform any HPDI ever made. I choose Mercury because they have always performed better than any others I have run. My self or my dad have run every outboard made from Tohatsu to Evinrude and I have good and bad things about each. However, I have chosen to run Mercury because they have performed the best and have taken excellent care of me with any work that was required. I know my post will look like a "plug" because I am a Mercury guy. but I will say that I have only been on their team for a few years, and still ran Mercury way before I was ever a member of the Mercury team.

I bought another boat last year. With respect to the power, I deferred to the best outboard mechanic I've ever known (who happens to also be a very good friend). He basically said what I thought he would say when I asked him about outboards to favor or avoid: "I won't let you buy anything with a Yamaha on it - you're too good of a friend.". He is not nasty about them - he even works on them often as rebuilding HDPI engines is a bread and butter part of his business. He would favor any other brand because of his years of failure observation, so his view is not just a Chevy/Ford argument. He said that my Pro X/S will likely never have major issues if properly maintained, which we will be sure to do.

I have a Z520 with a 250 ProXs. Go with a Mercury, you will not be disappointed.

Mercury...

A lot more maintenance with the hpdi. Had mine for 10 years and it was a great motor but I would get a pro xs.

Wow a lot of Yamaha hating going on here. I was in the business for 19 years and let me tell you both Merc and Yamaha have had their bad designs. Mercury Optimax was terrible we actually stocked complete powerheads because they were blowing so often when they first came out. Also Yamaha HPDI engines are very sensitive to the fuel you put in them especially if there is any water in the fuel at all. If you keep a water separator filter properly changed and buy gas at stations you know sell a good quality fuel then you should have no problem with the older HPDI's. Also the 2011 and early 2012 Sho engines did have power head failures and Yamaha is now replacing those at no charge. I know it off subject but the older carb model yamaha engines are bullet proof. I would also recommend speaking with a mechanic that services both engines to get a real answer and remember even Mercedes service departments stay full...good luck!

Not really hating... just the honest truth. I've seen ALOT more issues from friends with that model Yamaha than the Mercury Pro XS. Yamaha has made good motors in the past for sure, but the HPDI is not one of them when compared to Pro XS.

I have owned both. I had a 250 Pro xs for 6 years. I had a HPDI for 3, and currently run a 250SHO. My opinion is that if i was purchasing either a HPDI or Pro XS, I would choose the Pro XS. Better fuel mileage, better top end, and will stay together longer. I worked on a HPDI last weekend for the usual issues, exhaust tuner and filters. Also bad on the powerheads. I will say my Pro xs had about every issue known to man, however it was all warranty work and never anything major.

They all suck. I hate outboards.

If buying used (either make), it should come with a full service history and you should have the motor thoroughly checked out by a qualified mechanic.

I have been told filter maintenance is critical on the HPDI (as stated above). If I recall, there were two filters on the motor, and a water separating filter in the bilge. I had these changed annually on my HPDI, and I used RingFree in the gas. I ran that motor 6 years without issue.

Mercury hands down. I just went though this.