The weight of the vehicle and intended role both play important parts in the decision. If it's ONLY to get troops from Point A to Point B, then 10 tons and no weapons (meaning no turret) is likely to be the optimal solution. If more armor and/or cargo/infantry is needed than 10 tons can support, then I would go with a heavier APC and invest a couple of tons into weapons (forward facing MGs, SRMs, or LRMs) to provide some support for the infantry while they're disembarking and moving into cover, or possibly a turret with either a pair of MGs or an SRM-2 rack.
I can DEFINITELY see some utility in having at least one vehicle in the formation with a turret and a weapon capable of outranging most typical infantry weapons (SRMs, LRMs, or an AC). Note that an MG may not outrange infantry rifles, but an infantry platoon needs to at least think about the implications of charging an APC with an MG facing them. If your rifles are in range of a target, so is their MG. One is probably not enough of a threat, but a pair of them gets a bit scary for unarmored or lightly armored grunts. In later eras where BA is more prevalent, such light weapons are of little use, so you're better off with unarmed APCs and a well-armed escort (with a turret).
Then you have IFVs, which are supposed to hang around to provide fire support for the troops they just disembarked. In that case, a turret is a very good idea, and the weight of the vehicle will need to be at least 20 tons bare minimum, if not substantially more.