Like ArchAngel said, depending on your needs one may be more suitable than the other. That being said, I have both and do most of my work in Tradestation.
If you want an external data feed (the shorter your holding period, the cleaner the feed you'll need) you should go with Metastock b/c you can put a third party feed into it (although there are rumors of third party vendors making external data feeds available for TS). If you want to "scan" stocks (actually neither one is really all that great for real-time scanning) then Metastock is fair (I think you can have a universe of around 2,000 stocks or so). TS can scan monitor real-time indicators but you're limited to 500 stocks for now.
If you want an all-in-one package that's user friendly (for most people) then TS is good. TS is well supported and quite flexible in terms of what it can do. I like TS because of this. I also use them as a broker. I swing trade stocks and futures...any shorter of a time frame and I might consider a different broker...
All else being equal, I would choose TS. But again...it depends on what your needs are...
