Actually a combination of both would be ideal. If I had to choose though, I'd take the couple PR6 links.
If you go for only PR 6 links than you might caught as a link buyer. Better to have mixed up of both.
Just two PR6 backlinks and nothing else doesn't look natural. There's a good chance that the Google algo would notice this as suspicious and discount the links. It's better in the long run to have a mixture of different kinds of links with different PRs.
Two backlinks from a pr6 site is much more valuable than getting many backlinks from pr0 sites. However, to make your links appear more natural you should consider getting backlinks from sites with a variety of different page ranks.
If your backlink building is natural, there is no way to have backlinks with just one PR. Although the more backlinks with higher PR the better.
My choice will be to have many backlinks (related of course) from PR0 websites than having few unrelated backlinks from PR6, if they both are related then i will go with PR6 website backlinks
The key is to make your linking look natural. If you can get some good PR6 links, especially if they are on relevant sites, then go for it. But, also take some time each day(or week) to build some normal back links. Submit to a few directories. Submit an article online. Post on Digital Point. Comment on some blogs (don't spam). Variety is the key.... or, in investing terminology: "diversification"
I believe the general consensus here (and I would agree) is that you should not get too caught up in the PageRank of the site giving you a link. Just implement a good strategy of gaining backlinks and you will find that you will get a good mixture of PR sites linking to you and this will look natural to the SE's. Besides, you have more important things to worry about than what your backlink PR is...IMHO, of course.
From what I can tell you just need to get as many links from real websites as possible. As apposed to fly by night link directories etc.
You want to shoot for quality, quantity, and diversity. However, search engine rankings can, at this time, still be pushed around by sheer numbers alone.
I kinda agree with what most of us here have said; mixture of all. But in honesty I'm tempted to say that I really don't believe PageRank, has that heavy an influence on the "weight" of a link. Best advice I believe, is to make sure that your links are coming from relevant pages with relevant anchor text. PageRank aside.
I think one thing we could all agree on is that the more backlinks carrying search engine trust the better.
Don't worry too much about PR, whilst getting high PR links is good you should just focus on getting all the links you can... volume volume volume.