Hi.
I retail stoves and sell both radiating and convector stoves.
Traditional stoves are all radiating stoves, the flames heat the metal, the metal radiates heat into the room, simples.
Convecting stoves have air channels between the firebox and the outer case, air is drawn into the bottom of the stove and passes by convection around the firebox and emerges at the top of the stove, no moving parts, all done through heat convection. A convector stove does also radiate heat as it gets hot but it usually does not get as hot as a radiating stove meaning that furnature etc can be placed a bit closer, ie the minimum combustible clearance is less. Convectors are also slightly more efficient and most have a terciery burn system to re ignite the smoke and the particles within in, so more flame, more heat and less pollution up the flue. A win win win situation.
A room of 3m x 1.5m x say 2.4m tall ( so 11 cu m in volume) given poor insulation will want a heat output of around 1kw, yes one kw. to heat the room to 22C if its 1C outside.
If the room is a lounge area then dont go above 4kw, you will get heat spillage yes but not as much as you might think unless you put an Ecofan on the top.
The couple should buy the stove they like the look of the best, at the end of the day its them and no one else thats going to have to look at it. Yes convectors are nice, more efficient, bigger flames etc but they are at the top of the price scale so you do pay for the benefits.
If the stove is an inset the Esse 301 and Aarow I400s are nice convectors, radiating inset then most are 5kw, ( Aarow Ecoburn 5 Plus Inset), the Broseley Evolution 4kw is a radiating inset but is a dedicated wood burner. Personally I don't like the Broseley as its ash removal is tricky.
Freestanding, then convector Morso S11/42 (4kw) stands out a long way in a narrow field, most convectors are 5kw -8kw, radiating Esse 500 (3.8kw), Aarow Ecoburn Plus 4 (4kw) and Acorn (4kw), Morso 1430 (4.6kw) would be my shortlist of good and best quality stoves.
Hope that helps.
A