I would recommend electric in-floor radiant. I would also strongly recommend a Schluter Ditra decoupling membrane under the floor tiles and over the in-floor radiant to reduce the risk of cracked tiles. I would also recommend an epoxy grout that is both waterproof and will not stain or discolor.

Please see the links below:

http://www.suntouch.com

http://www.schluter.com/2080.aspx

http://www.mapei.com/public/COM/products/kerapoxytds_ea.pdf

Here is a link that might be useful: Sun Touch

Since you have gas, you might consider some sort of wall-mounted gas heater, vented or ventless.

Your plan sounds great.

I agree with weedmeister, a smallish gas wall mount should easily handle 120 sq feet. I'd also get vented rather than unvented since the room is mostly closed up. Your placement options may be a bit limited by code requirements for the exhaust duct, they often state minimum distances from opening windows and doors.

Heat using any kind of gas will likely be cheaper to operate than any kind of electric radiant heating.

Thanks for the ideas, I was recently looking at this http://www.wayfair.com/Mr.-Heater-20000-BTU-Natural-Gas-Vent-Free-Wall-Space-Heater-MHVFB20TB-NG-MXH1055.html. It is so cold in there I wonder if the heated floor would be enough to keep the room warm... my brain is so torn LOL!

Radiant floor heating will work. The main issue is that it will take quite a while to heat up.

Having better insulated windows (

Having windows with a U factor If you use forced air heating, the space will heat up quicker than radiant floor systems. If the slab is not thermally broken/ isolated from the ground or the windows are losing heat, the heater will be on quite a bit.