BeccaB congrats on your approaching milestone birthday! That is a big one. I don't have a Garmin but do have a BB. Since I can't compare I would suggest a GPS unit for navigational purposes if you get lost a lot.

My BB has the app MapMyRide on it and it also has a GPS navigational system that is a paid by the month feature so if I got lost I could use it to find my way through the GPS, mapmyride does not give live mapping data visible on the phone, it just saves the route and you can view it on your computer. This might work different on a Droid or iPhone. MapMyRide offers a live feature but I think it works on a computer, as in you let people know you'll be riding and they can "follow" you live on their computer. Handy if you are alone and something dreadful happens, at least people would be able to find you. If you have smart friends or family that know you use this application and you ever got abducted you could turn it on and you could be found. I know, my mind works pretty out there sometimes.

No matter what navigational feature I use on my phone it sucks the life out of the battery so for long rides that could work against you.

For the purposes you are wanting to use it for, I'd go with a GPS only unit. Plus if you don't want the absolute latest and greatest phone most phone service providers offer an upgrade pretty cheap as long as it's not the newest phone out there. When I first got my BB three years ago my provider was going to charge me $100 for it, I squealed that I've been with them for over 15 years and I could get one for $50 from another provider. I ended up with a free Blackberry. It pays to speak up if you have been a loyal customer.