Private always trumps shared proxies, hands down, though depending on how much heavy work you are going to do via SB, that will paly a big part also as you would probably burn out your private proxies pretty quickly.
The usual routine seems to be to grab a good selection of shared proxies for your scraping needs, and then only use the private ones for comment posting, pr checks etc... Seems to be a formula that works pretty well, and will save you some coin.