Do you want a glass cannon that can rip holes through a team, or a defensive fighter that can shake off being hit by a plane? That is the question!
Rampardos has a phenomenal Attack stat (165) and a complimenting movepool to boot, but some people are turned off by his fragile defenses and middling speed, which makes for a terrible job at sweeping, which is the only thing that Rampardos is good for. Rampardos is at his best if you can baton pass agility onto him, so he's sweeping-ready.
Bastiodon has phenomenal defensive capabilities (168 base Defense, 138 base Special Defense) and also has a complimenting movepool full of tricks and tactics. Bastiodon's drawbacks are its weak offensive stats, low base HP, and carrying the burden of TWO x4 weaknesses. When compared to other Rock-type walls, like Probopass, it seems mediocre. But compared to Shuckle, its a dream come true. Bastiodon is at his prime as a walling switch-in.
I can't make the decision for you. The two are like day or night, so judging by this, you'll have to pick which battle style you'd like more in your prehistoric pal!