Actually it helps with both...just not for long. You have to continue getting those fresh blog links because the old ones will be moved down the list into a deep black hole. In fact it really is like a black hole. If you are fast enough you can stay out of it (by getting links all the time) but if you are too slow you will be sucked in.
Blog commenting is not too much helpful, but can't ignore fully. I have a matrimonial website & 3 or 4 links through blog commenting.
You are definitely right. I do think that blog commenting really helps in gaining traffic and Page Rank.