a combination of both may also be useful, using only follow links may seems suspicious so a combination will seem like a more natural stratergy
agreed... it defintely looks natural that way and I think that is THE most important thing to consider. Much like using different keywords as well to appear natural.
Like many people have written on this forum, page ranking is virtually useless and will not help you rank for your targeted keyword. What you should be looking instead is how to increase your position on your favourite SERP.
It's hard to get all the links from dofollow forums, blogs and social booking sites, since most forums, blogs and websites are nofollow. If you can find many dofollow sites, then priority must be given to dofollow.
If you are after a successful, popular website, any link is a good link, the more the better. Diversified keywords and types of links (forums, blogs, authority sites, .edu domains, foreign language, etc) will only serve to benefit you. Selling a website or attracting advertisers though, the more PR, the better! Noone knows 100% sure how to get PR and I doubt Google will ever release their secrets. Good content and plenty of backlinks should get you well on your way. If you have douche content, you don't deserve to get high in the SERPs or high visible PR. If you do manage to scam it, it won't last.