For what BB is best for you google them and review them. However I find myself in the same place. I have a storm 2 which is dated waiting for BB10. Just get the cheapest phone to suit your needs. Both should bridge well with the PB.
From a practical standpoint, you can use a BB phone that has EDGE.
I have a BB9000 (3G, but I have 2G/EDGE on TM) to PB combo which has slightly slower bridge/tether then my wife's BB9900 (4G on TM) to PB combo.
The limiting factor is bluetooth, same on most BB's.
And, you MUST get a BB phone which has, or can upgrade to OS5
How much is a Lumia 710 from T-Mobile? I'm using that to tether my PlayBook, I much prefer it to my Curve 9300 that I could bridge with. Also, I prefer to keep phone purchases of the same platform 2 years apart, alternating platforms on a yearly basis gives me more variety and I don't suffer from the feeling of incremental upgrades.
I am using a 9780. It works well with playbook. It has double the capacity of the 9700 and a better camera if that is important... Other than that they're practically identical.
There is a thread that discusses this question. It concluded that both were good but the 9780 would be better. Check under the 9780 At the top of the screen.
9780 only runs os 6, if someone is running 7 on one. Let me know! I find the 9780 better than the 9700. I just received a brand new white 9780 bold from my Cdn provider and after trade in credit, I only paid $25 for it ( I traded in my black 9780)). But extended my contract to 24 months with same plan I had before. Not worried about penalty when bb10 is available as I can sell it under warranty.