My view is this: start with a decent sub, and then you will be able to appreciate moving to a great sub later. That's essentially what I did. Especially if you don't have the finances for the great sub now.
That way costs more money in the long run. If one buys new equipment and later resells it, one almost certainly will lose money. A lot of people have wasted a lot of money doing things that way, and if they only had saved up for something really good, they would have better equipment for their money than doing upgrades from lessor equipment. If one can afford it, one should buy a subwoofer that is good enough that one never feels the need to upgrade it. In my opinion, the ONLY reasons to go with a lessor sub are if money or space are lacking for the best.
To sgtpepper9:
If you can afford the Ultra, just buy it. If it has more capability than you need, it will do no harm, but if you buy a lessor one and need more bass, you will be out more money in the long run. And if you like a lot of deep bass, there is a good chance you will want an Ultra. Also, when you push a subwoofer to its limits, it typically distorts more than when operating well within its limits. So, even if the lessor one would give you enough bass, it might be less distorted with a better subwoofer, if the lessor one is pushed to its limits. Since you say you can afford it, just buy the Ultra. Otherwise, you will probably be buying one anyway in a couple of years, and will have wasted money.
If you are married, your spouse will probably like it if you have no plans on ever upgrading. I have 2 of the old SVS CS-Ultra subwoofers, and I have no plans on ever upgrading them. And I have Aurum Cantus Leisure 2SE (U.S. version) speakers (all channels), and I don't plan on ever upgrading them. My wife is very happy about that. And it all sounds great, and the speakers are beautiful. Here you can see pictures (of the newer version), though the pictures do not do them justice:
http://www.kellsieavdesign.com/products/Leisure2SE.htm
Anyway, just buy the Ultra and be happy that you have a great subwoofer.