Oh, I understand now. If your goal is to have a sealer for dedicated use with shrink wrap, the round wire is probably best, because it will get hot along a narrow line to efficiently melt and cut the shrink film in one step. For larger volume packaging or if you expect to use the sealer only with heat shrink film, this would be the machine to get. You would be able to use it to just seal other types of plastic bags by reducing the heat interval, but the narrow seal from a wire will not be as sturdy as a wider seal from a ribbon.
For hobbyist or general home use when you want the machine to seal a variety of plastic including shrink film, I think a flat ribbon sealer is the more versatile choice. It makes a wider seal (2 mm to 5 mm wide) that will be more reliable when you're looking to seal a bag so it will remain securely closed. A flat ribbon sealer can also be used to melt and cut shrink wrap -- a 2 mm flat ribbon sealer is what I use. I just set the heat interval higher so the film melts completely through.
ETA -- I'm not quite sure why they're saying you can't cut shrink wrap film with a ribbon sealer -- that puzzles me because mine works fine to seal and cut shrink wrap bags. (Here's where I found that mention:
http://www.uspackagingandwrapping.c...e-Sealers/Hand-Impulse-Sealers-Flat-Wire.html )