What's the point of turret if 221 can not out run M3? Stug will lose to T34 because T34 could out run Stug.
221 is not 223, it can not secure resource or detect vehicles, it can only dispatch medic. And the problem here is OKW's starting unit could dispatch medic, too.
If you want to count other potential factors. What could be more deadly to retreating squad? A M3 with a squad of flame thrower CE or a 221? And a M3 with a squad of PTRS penal or Guard could be deadly to Ost's Halftrack, too.
A question here: If you are a SOV player and relic plan to increase M3's price by 15 fuel and remove its ability to transport infantry in exchange of the ability of dispatching medic, will you be OK with that?
Turret is still a turret. Useful for if you're flanking a position (say, an mg or infantry in cover) and then don't have to waste time turning. There's more things the 221 can fight than the m3.
The point is the 221 has the potential to be a 223 and have all the utility associated. It's not a 223, but it can be made into a 223 and thus that potential should be taken into account.
A loaded m3 is better against retreating squads for sure, but at the same time that also means that you have a squad in there. The fact that it can do that is an advantage though. The 221 is still quite good against retreating infantry, especially since it seems a bit more manueverable and has a turret but it does obviously lack the firepower that you could have with the m3.
If you want to expand that comparison to making the m3 into the exact same as the 221 (in addition to medical supplies having a turret, the mg42 on top, the ability to upgrade to be a cahce and have smokes) then hell yeah, let me have that. Sure, it'd be a tradeoff, but I freakin love the 221.