I don't see how redirecting that domain to your site is going to help your SEO all that much, unless that site had a lot of existing links pointed to it. People aren't going to link to your pages through redballoons.com; they're going to link to your domain now. I would be very surprised this would get your from rank #21 to page 1 on the results pages (unless that domain had a bunch of relevant links pointing to it).
If you're building a brand name at your domain now, buying domains with the keywords you're targeting and redirecting those to your site aren't going to help much. I wouldn't bother.
I can think of one use for buying this domain... if you're doing PPC, you could try using redballoons.com in your display URLs instead of your current domain and see if you get a higher CTR and/or more conversions.

