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Thanks. I will just go with 301 and see how it turns out.

Personally, I wouldn't go back and forth. I'd pick one and push to the other depending on your goals. If you don't abuse you shouldn't have a problem IMO. You'll need to be extra careful if the sites are registered to the same people and sit on the same c-block. Link where it makes sense, link to other places, and don't be too heavy-handed and it should be valuable if the sites are disconnected enough (not in the situation I described above re-registration, etc.) and it's actually good for users. Referral traffic should help you determine that.