I like the fact that you've considered the reward system for sharing, although I'm not sure why it would have to be one or the other.
There's really no downside to offering share functionality but I've seen many cases where it's just an afterthought that's there if someone wants it, but doesn't really offer any incentive to do so.
I like it when people prompt sharing with some of the work already done, such as "Tweet this: Here's 140 Characters already setup for you" (my words). It provides some context for what you'll be sharing, and if you're active on Twitter, just does a nice job of teeing up the reward (feeding your Twitter feed beast).
On the referral ID side, how about offering some sort of immediate benefit just by making their first share? Like offering $5 credits (I'm making up a number) just for sharing. Sure, you'll get some people that take advantage of the system, but you'll also get people actually helping share the word.
Happy to answer more questions around specifics.