I would second what Matseb2611 said. If you plan on purchasing a lot of Resource Packs from the store, you get a good discount. And, when you buy from the forum store, the artists get a lot more of the money (Steam takes a good chunk). And from what I've seen, it is a LOT of work (hundreds of hours) to create, say, a new tileset, so the artists should be well paid for their hard work.
Also, some Resource Packs include neat little bonuses only in the forum and RPG Maker store versions, as an incentive to buy from the store. For example, the Rural Farm Tiles include character pieces for the Game Character Hub.
If you have the Steam version of RPG Maker, you can use the Resource Packs from the store. Just drop their contents into a new sub-directory in C:\Program Files (x86)\Steam\steamapps\common\RPGMaker\dlc. That is also where the Steam version drops the Resource Packs you purchase.
However, the Member+ resources are also nice, and you can legally use them in commercial and non-commercial games. I think the ReStaff resources can only be used in non-commercial games without purchasing a license. And you can't get the Member+ resources elsewhere, although I've heard they've been trying to figure out a way to package old Member+ resources up and sell them.
I'd say it's worth it if you want more exclusive resources and want to support the artists who make all of these resources.