A 50w would work too and I'm suggesting that because those are normally easier to find :) And in many cases, cheaper too.
I got a new submersible, adjustable heater from ebay for 10 bucks with free shipping. So far it works but I haven't checked the temperature yet so I can't really give you a solid review. The only cons are that since it's over seven inches, it's a bit obtrusive and the cord is pretty short.
If you're curious though: http://www.ebay.com:80/itm/ws/eBayIS...m=170717496906
Edit: Forgot to add that Im currently using the above in a 2.5 rectangular tank. It may actually be hard to try and fit a heater shaped like that into a bowl ^^;