If only one choice - convection microwave, convection oven, regular micro, conventional oven?
IMO only a good convection microwave can replace a normal oven. They're not cheap, but I often use mine rather than heat up the big oven for something small *. The heat is a bit too much from the top for some baking, such as tall cakes, but it's OK for most things and there are workarounds like preheating with a pizza stone or covering a cake with foil. 

I've actually got two convection microwaves (both Panasonic) because that's my main cooking option in the campervan when I've got electric hookup. I liked it so much I got one for home.

The biggest downside may be unfamiliarity - your guests may struggle to use the right combination of settings and accessories/dishes. They're also rather small so you won't be cooking a roast dinner in one. 



* combination mode is also very good for some things but take a little getting used to