I would highly recommend going to the demo skis to anyone who is actually trying to get better. The main reason is that they are trying to make sure you like the skis so that you'll want to buy them. Therefore they have a reason to keep them tuned well. The rentals get almost no love because they want to keep the skis for a couple of seasons and can't tune them often enough. Plus, rentals that get some damage will end up back on the rack, whereas the demos would get removed from the rack if they were damaged.
I had the unfortunate experience of having an airline trash a pair of skis on the way out to Utah many years ago. So I got the highest end rentals they had. They had skis that I had skied on as demos the prior year, so I knew how they *should* ski. However, they were trashed, and skied like they were trashed. Over 3 days of rentals, I went though three pair of skis, and they were all junk, even though they were top-end skis.
Then, a couple years ago, on a trip to Whistler, I decided to rent mid-fats for a couple of days, because all I had was narrower skis. After the first day, I took the skis back to the renatl shop and forced them to grind and tune them while I watched. It made a big difference.
All of that said, if you ski more than about 5 days a year, I would really recommend that you look into purchasing skis. Maybe look on ebay or something, for new skis that are from last year or the year before, that you can get cheap. It helps a lot to be familiar with the equipment you're skiing on, so that you don't spend a couple hours trying to figure out why things don't feel good, only to have it be that the skis are different than the last pair you were on (note that you probably would never realize that it was the skis, but just that you didn't feel like you were skiing well). With your own equipment, you will also know that they are in good condition, and not have to worry about trying to ski on someone else's sloppy seconds
Also, I'm assuming you have your own boots already? If not, you should, at the very least, get your own boots, and rent high quality boards.