As you know, there are a few different styles of cell phones. Bar phones, flip phones, and slider phones. I've owned all three styles at one time or another so I'll offer a bit of insight on each.
The pros of a bar phone are there durability, also, they're typically the thinnest of the three styles. The cons being that they constantly dial phone numbers if left in your pocket without the key lock!
The pros of the flip phone is their compact size, the fact that when they're shut, the screen is protected. The cons for a flip is their thick size when folded. They're more delicate as well due to more moving parts.
The pros of of a slider phone is the key pad is hidden when not in use. This obviously means you won't inadvertently call someone when its in your pocket. The cons are similar to the flip in regards to delicate moving parts. The screen can also scratch if left in your pocket or purse.
I currently own an LG Lotus, flip style phone. Obviously I'm biased, so my choice is a flip style phone. I think they just look more stylish and compact. The fact that the screen is protected when the phone is shut is the biggest positive for me. So, get to your nearest cell phone provider and play around with the three styles and chose accordingly.