What Is The Best For Me, Is It A Pda Or Sat Nav. Please Advise ?

I dont own a car, but I rent one once or twice a month. I ride my bike a lot within London, and I do a lot of walking.
When driving I tend to get lost a lot, but because I dont own a car I would like the SAT NAV or PDA to be cost effective and very portable. Please advise on what the best one would be for me for ease of use, cost effective but with some features
Thank you in advance for any comments received.
P