I have neither (yet!) but,
TiPeN: small, ballpoint/rollerball refills, screw on cap, cap in grade 5 titanium, no clip
Ti2 Pen: larger, hi-tec-c refills, pop-on cap (grade 2) with o-rings, optional clip in grade 5 titanium
Both: bodies in grade 2 titanium
(Grade 5 is an alloy that is stronger than grade 2, which in turn is basically pure titanium.)
Personally, I'd probably have more use for a Ti2 Pen, having a dozen of G-TEC-C plastic pens...