Both are accepted but my spontaneous interpretation of this sentence is that it's not a noun but an adjective. If we speak about people being professionals as a noun, we usually use proffs. It's a bit colloquial, but very widely used. Otherwise I might use yrkesmän.
PS It's ett proffs, proffset plural proffs, proffsen

Instead of being "pro-feh-shun-el-la", it's "pro-feh-hwun-el-la", with the swedish "sj" sound. It's a very unique sound, so if you can't pronounce it, at least say "hw" or something like that.

If I'm not mistaken it's: /pɾɔ.fɛɧ.ɧu'nɛl.la/

It is pronounced that way.

Hmm, checking frovo I found http://www.forvo.com/word/professionella/#sv -- poor audio quality http://www.forvo.com/word/professionellt/#sv -- professionellt http://www.forvo.com/word/professionell/#sv -- professionell

I am going to go with (profefhunella)

The "ss" sounded almost like a "k" to me, which makes perfect sense in a language where the "k" often sounds like an "s"... :)

For me the Spanish "j" is the closest.

Singular (utrum) and plural.