Electric floor heating is most likely the right solution to warm a 700 sq ft area (only about 400-600 sq ft will actually be heated). Hydronic systems are typically used for whole-house systems, although you may want to get a quote from a hydronic manufacturer just to compare (700 sqft is still a large size space and you may find hydronic systems to be a competitive solution, even after factoring in the higher installation costs of a hydronic system).
To select an electric floor heating optionn that will work as a primary heat source, make sure you choose a system rated at 15 watts per square foot. Some systems are only rated at 10 or 12 watts per square foot.
Our Tempzone systems (http://www.WarmlyYours.com)
are rated at 15 watts.
A 15 watts system will work very well as a heating solution, although you need to make sure that you're not installing it over a cold cement slab that will suck the heat down. Additional insulation may be needed, and you should talk to your floor heating manufacturer about specifics.
WarmlyYours provides heat loss calculations and will tell you how many square feet need to be heated for primary heat.
Hope this helps,