Assuming you're talking about hydronic (not electric) radiant vs. baseboards...
And assuming you've already got a boiler that can handle the additional load...
Is the slab insulated?
If not figure a cost adder for 1" of XPS and a 2" quickset over slab (+ labor) over the existing with radiant. It'll be cushy, but figure on losing 3" of headroom, and you'll have some efficiency gains by being able to run very low water temps. If it's already insulated below and head room is an issue Warmboard(tm) or similar might be the ticket (but probably as expensive or greater than a thin slab.)
If not and the rest of the structure is reasonably tight, 1/2" XPS between sleepers for a finished flooring (or just a vapor barrier under some reasonably thick carpeting undelayment) and some baseboard should suffice. If you want to regain some of the higher efficiency of radiant, doubling the length of baseboard that would have been required for 180F water and lowering the water temp to 130F is usually cost-effective.
Whatever way you go, do a full-on manual-J type heat loss calculation, and make sure you neither under or overdo it on the radiation or heat source end.
If you're talking electric somebody else needs to break it down for you... (it's not my cup o' tea.)