I know on the original Saker 7.62, shooting with the 5.56 endcap vs. the 7.62. The 5.56 was quieter, even against the 5.56 model.
Heard this time and again. It's a bigger can so more volume equals better suppression in general, and with that endcap and all the other shit they make fore the Saker, it's hard to go wrong. It's a very well thought out, designed and built can. Stellite is a bit overkill IMO but hell, I guess it'll never wear out in my lifetime. Saker is one of the best cans available and I doubt few here would disagree.
Generally, a 7.62 can has more volume and with the larger opening lends to a deeper, throatier sound. It suppresses as well if not better, depending. I started with 7.62 cans.
5.56 cans generally produce a higher pitch and depending on conditions that can make it sound quieter. They're usually shorter and lighter. IMO, dedicated cans are worth it for several reasons.
I have the 5.56 Saker and it's one of the best military cans out there, solid simple QD and very well made. If weight is a problem, get a TBAC and watch the temperatures (no continuous mag dumps). KAC QDC is another I can recommend now but it's an expensive setup when you can get the Saker for less and it may even be quieter than the KAC, but the KAC QDC has a ton of features no other can has. If you have a KAC upper with the mounts already, this is the can I'd probably just go ahead and get.
Can't speak for the Harvester, but I do know it's a hunting can and I generally don't buy hunting or multi use cans.