I think the mineral glass used on Seiko 5's would be fairly durable. They can last for years or decades. If you look at the tons of vintage Seikos floating around, almost all of them have still have their original crystals. But you'd also see a good amount of scratches on them. Mineral glass has a good combination of hardness, shatter resistance (they're probably not as brittle as sapphire), and heat resistance. But they're definitely not as scratch-proof as sapphire.
Again, whether you get scratches on the crystal will depend on how careful you are with your watch. If you wear it while building a back porch, you're going to get it roughed up in no time. But for normal wear around the office and the house and even for casual sports, the mineral glass should be more than OK.
If you want a Seiko with a sapphire crystal, you'd have to go for one of the higher-end models -- perhaps a kinetic, solar or quartz. Some of the more expensive models also come in titanium cases that are supposedly very scratch resistant. Or consider getting a Citizen -- many of the Eco-Drive models come with sapphire crystals and titanium cases and are reasonably priced.
If you're interesed in doing more Internet window-shopping, check some of the net dealers for the different types you can get, especially the Asia-only models. Here's a list of dealers from the FAQ: