I spent one night at the Ren Wien last year. It's decent, but the rooms are spartan, even for a Germanic country. The plastic bowl chair and small plastic desk were nearly useless. But, I thought the location was fine. It's a 50 euro cab/car service to the airport, and the Schonbrunn is a 10-minute walk. Most tourists to Vienna are probably going to visit that palace. It took me an hour just to walk the gardens, and I didn't make it up the hill to the Gloriette. To do everything inside and out, that's a solid 4 hours, at least. So, saying that the Ren Wien is away from the touristy stuff isn't completely accurate since you'll be out that way for half a day during any trip.
One general note about Vienna - you don't appear to flag down taxis there. You seem to have to find a marked taxi stand, or a hotel with a stand.