Side carb:

Usually means piston ported. That means less power than a reed valve engine. But 3W makes a side carb reed valve. Side carb is usually wider, so if it's warturd, it may not fit in the typically narrow cowl found on those types of planes. Side carb is easier to choke though. If you're using your thumb to choke, the side carb is usually sticking out the cowl or very close to the cowl so a small hole will make it easy to choke. Shorter though. If you have a short cowl and you're not good ad cutting holes in the firewall, the side carb will usually be shorter than the rear carb.

Rear carb:
Usually means reed valve or rotary valve. Both are more powerful than a piston ported engine. Longer engine though. You use long standoffs or cut a hole in the firewall. Usually more expensive than a piston ported engine. More moving parts, so more chance of failure.

Probably a dozen more reason why you should or should not use each particular engine. I think the choice comes down to affordability and application. What fits your budget? What fits inside the cowl?

Modern gas engines are light years ahead of your dads old chainsaw from 1965. Pick one and go for it. Not much you can do wrong except forget the oil or run it too lean.