I have a Blackberry (albeit an older 8703 model), and I love it because it has voice recognition technology software that I downloaded from the internet. That way, I can press one button and it'll type in stuff for me, i.e. addresses on Mapquest so I can find my way around. iPhone is great too, but if you can hold off (or have to save up the cash like me), I'd say wait until early next year when the new model is out. I am very much wanting an iPhone simply for the apps, and because it seems more intuitive.
As for Bluetooth, I suggest getting two headsets, so that you'll have a spare if one dies. I've found dirt cheap ones on the internet (i.e. eBay or newegg.com, etc), and I found they were a lifesaver, especially while in the hospital/rehab. Also, when I'm up on crutches, I hate hate HATE not having my Bluetooth adapter - I'm trapped in one spot without it on!
When I turn my Blackberry on for speakerphone, it has very clear sound, and people can hear me very well. HOWEVER - if I happen to pick up the phone, and the mic is next to my face, the person on the other end can't hear a word I'm saying. When you do shop around, check and see where the mic node is, and how large the mic area is (mine is the size of a eraser shaving!).
Also, check on eBay etc. for secondhand/last year's model phones - that are UNLOCKED, preferably. Of course... if you say that you're bringing the phone you purchased abroad and you want to use a SIM (the little card inside the phone) from the country you're going to, you generally won't get any flack. Anyway, I use Sprint, so that point is moot.

I like the plan I'm on.
If you work for a company (usually larger ones), or your significant other/kids do, check and see if they have a deal with a large carrier. I know that my old company worked with Verizon who gave a 15% discount on monthly plans, plus a 25% discount on phones. If your company uses a BES (Blackberry Enterprise Server) or equivalent, you might be able to hook yourself up with their network and get your email on their phone, plus some support (sometimes!) from whoever administers Blackberries.
Decide what you're going to use on your phone. I use my phone as just about everything, so I have a plan that scales for that. If you're price conscious, make sure to shop around with the major brands. For example, a iPhone through AT&T will REQUIRE you to add on the data plan, regardless if you'll use it or not. The plan usually costs around $80/month, I believe. If that functionality is important for you, go for it. I've seen dirt cheap plans as well for other phones, for when you just 'occasionally' chat on the phone.
Finally, always make sure that when you sign the contract, you have a 'trial period', and you make sure that your phone works in most important places, like the bathroom in your house! If you have the time, check and see if 'you're good' everywhere you typically hang out. So so so many people I know have cell phones that are dead in their house, or at their company, or whatever. No good.
I used to do specialty cell phone stuff for a company I used to work with, so sorry if this is overboard! It's always hard to guess what sort of cell plan/phone/devices will work best for a person without personally knowing them.

Good luck!
eta: also, make sure you 'test' phones, to get the feel in your hand. I prefer a thicker phone, which is one of the turn-offs from the iPhone for me personally. And why I love my old phone.