The only advantage to getting a convertible for your 5 year old at this point (unless you wanted to rear-face him, which I guess at his size could be an option but not many people in the US rear-face kids that age) would be to pass it down to your younger child later as a rear-facing seat.
A forward facing harness is a forward facing harness, though, and when used in that position there is zero difference between a convertible used forward facing and a seat that only faces forward.
Normally I'd say you'd get more use out of a forward-facing only seat because they tend to last longer, but your 5 year old is so small there are several options for convertible seats that will be fine until you are ready to put him in a belt positioning booster (opinions vary on this but my opinion is that it's best to keep him in a harness until he hits the 40 lb mark; at his age and size that probably won't be until he's around seven, and he should be more than old enough for a booster by then). I don't think you're going to really need the higher-capacity options you can get from a forward-facing seat, and generally speaking a dedicated booster is usually a better fit for most families, cars, and children compared to a combo harness/booster seat used in booster mode. So really what I'm saying is: Get the seat you like that is tall enough to get your DS to the 40 lb mark, and don't worry too much about whether it is a convertible or a forward-facing only seat.