Its hard to say you have asked a kinda general question. To raise in the serps I would say find out where your site is in the serps then find out what the competition is for where you want to jump too and then see the quality and quantity of there backlinks and i think that would help you make a decision. Also i would shoot for bookmarking sites with pr1+ (no pr0) Tbh it boils down to many things even with the bookmarks. If its a good bookmark and gains popularity then it stays at the top more and means google spiders it more often and ends up on your site more often. So its not just quanity.

Well if its back links you are looking for both the ways are good enough...

if it were me, i would prefer the first one

both can be effective... But you should also consider the votes of your members on your bookmark

Neither. Let the world do the social bookmarking, you work on building the quality service that attracts them.

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If this is the case, I think the number one is better enough to rank at SERP rather than the number 2 since it is in high page ranks.

Before I'd have gone with the first option but lately I'm tending to go with the latter, then build up on the sites with higher PR.

for me the second option will be the best to increase the SERP. But all depends upon the number of votes given to bookmarks by the members.

The 1st option is much better as the pages will get indexed more quickly and can give you backlinks faster.