Buzzer is the same. I've used several of each model and can detect no difference in loudness of buzzer -- they sound exactly alike.
I prefer the second model because it displays a shot string in a more helpful way. This is important when practicing because you quickly know where you are loosing time and, thus, where you need to focus practice. I also think the controls are more intuitive (note: that is a RELATIVE not and absolute statement).
We use a lot of first generation models in clubs around here and they work well (except for the tendency for the battery door to fail). But, I prefer the second gen. It is easier to hang onto with it's smoothed profile.
My personal timer is a model II and I'd buy it again if I needed another.