For a good working "mechanical pencil" with a very useful features:
Pentel Twist Erase .5mm is tops!. It has one of the inch long erasers that twist out the end. Since I erase a lot, a good sized eraser is a must and I hate having to remove caps and stuff to get to the eraser!!! I do NOT like to have to carry a second eraser. Those dinky little 3mm wide erasers in many pencils just dont cut it. Plastic body, with rubber around grip area. Full metal end piece doesn't wobble even after a lot of writing/dropping, etc.
The other recommendation for a mechanical pencil with a LARGE eraser is the Papermate Titanium. It too has a LARGE twist eraser and can be had in .5 or .7mm (maybe even .3mm, but I consider that size too fragile for real work). It also has a floating end piece (you can push in the end so it doesn't stick out in your pocket). The downside is that the floating end make s it feel wobbly when writing hard. Also has rubber around barrel area for grip.
Obviously both come in different colors, although I always choose black or blue. I like the Pentel better as it seems to be sturdier over the long run.
These recommendations are from last year's college curriculum consisting of TONS of calculus, physics, and chemistry. Hence the need of a REAL eraser!!.
I tried more expensive/fancier mech pencils, but these two are real working tools. I rarely use pens so I can't help with that.
Oh, yeah, the Papermate Logo4 is also a good large erasser model. It's bascially the same as the Titanium without the retractible end.