If you're satisfied with the company you're currently with then I'd definitely stick with them on their more advanced plan. As for anticipating the upgrade, it depends on your traffic and your website's resources. Like someone pointed out before, certain mods./plugins use a lot of resources yet will function fine if only a few users are on your website but once a lot are on you'll start to notice performance loss and even account suspension if it got too bad so you should monitor resource usage with your current traffic and estimate when the time is right to upgrade.