This topic comes up quite a bit and as always, it's all about opinions. It really comes down to specific coaches, how they were treated etc rather than a blanket - which is better - newer or older and "nicer". The crux of the issue is defining "nicer" or better and that has so many variables you can't possibly get a definitive answer without scrutinizing each coach. You could find a good deal on a used Prevost or Foretravel that was really mis-treated or already has failing or ready to fail components. But based on facts there is this; Age is the enemy of anything, and I don't care what folks say about owning a high end coach, at 8-10 years old stuff is not only becoming obsolete and out-dated, things will start to fail. The true answer is this - no one can tell you what to buy. You need to examine each coach and find what YOU like about them. Once you narrow it down I am sure specific questions about components etc can be addressed but all you are doing at this point is asking for a bunch of opinions. And you'll get them. If it were me, and I knew I would keep it for 15 more years, I would opt for the nicest, newer coach I could. In 15 years that 3 yr old coach will be 18 and the 8 year old one will be 23. You think about what a 23 yr old MH looks like today vs an 18 yr old one and think about the condition. 5-10 years difference in MH's is a lot of use, abuse, weathering and time for things to fail and become obsolete. You think about any big purchase you make. Houses, cars etc and you tell me what needs addressed in that 8-10 year mark. What the components are like, finishes etc. Time is the enemy. And cars and houses are typically built better than RV's and subject to less abuse.
On edit: BTW I would avoid the Thor ;)