I run a sidesaddle with slugs on the left side of the receiver, and a buttstock elastic loop setup on the right of the stock for my less lethal rounds (since they weigh nearly nothing). The noticeable weight is on the left side of the receiver.
Receiver location.... any extra ammo is much quicker coming from the receiver than buttstock mounting. Always. Because elastic buttcuffs are not precise compared to hard mounts, and hard mounts need a receiver. Yep, the right side of the buttstock can be very quick to the open ejection port, but isn't predicable if you are talking about elastic loops.
Weight? I don't care.
I run a 590 8+1 that weighs a lot. That is the one I mentioned with extra slugs on the sidesaddle. Its heavy. I don't care.
I tote a wood stocked M1A at work that weights a huge amount. It is a heavy bitch but I like it for its effectiveness.
I run a HBAR M4 with an Aimpoint CompM4s that weighs a lot more than your ordinary AR. Again... I don't care.
I already wear about 25 pounds of extra shit on my body.
An extra ten or twelve is unnoticeable in the moment.