As stated above, Bell and Giro are the same company now, and their helmets at comparable price points are very similar. They have different trade names for their adjustment systems, but they are basically the same, also.
The two brands have some fit differences, though, in the basic shape of the helmet. One of them is a better fit for a more round head (when looked at from above), whereas the other tends to fit more oval-shaped heads better (also from above). But for the life of me I can't remember which is which.... (You'd think I'd know this, having once owned a Giro helmet, and now having a Bell.) So if you try them both on, in the comparable size, you might find that one has more relative space on the sides, where the other has more relative space at the front and back.
In terms of head protection, I think that all approved helmets are essentially the same, regardless of brand or price level. So you are really looking at comfort, style, air-flow/venting, and weight, as deciding factors.
