Are the two models standalone or is one a handheld and one a PC connected unit?
I've ended up with 3 electric labellers, a dymo handheld which is ideal for labelling cables/plugs and the like, a battery/mains PC one that does the same sort of job but hooks up to the laptop for input and is more versatile (handy for doing labels in the garage for drawers etc), and a mains powered dymo 450 which is used for letters/packages, and has a label size that is perfect for Ikea's storage boxes.
They're all good for their specific use, but the handheld one is more useful for small labels like the cabling (as you can carry it with you), and the desktop one obviously for normal labels, with the one with the built in battery filling a niche between the other two.