Hi,
I would recommend doing a BYC Search on both. Looks at what others have to say. Then do a Google too. And don't forget to look to see if there is an established breeder's club.
So far the comments on here look to be pretty accurate. One thing I'd add is that Barnevelders are very rare. So if you do get them, for the bird's sake don't get them from a hatchery!
As for Welsummers, I breed and show them. I'm working on both APA SOP's and improving the size and color of the egg. I have one hen that is laying a very large, extremely nice, evenly colored dark egg without any speckles.
You can see my birds on my website. Just click on the applicable album. I'll be adding some Barnevelders to my flock in a couple of months. I'm focusing my time and efforts on my Ameraucanas for the next 2-3 years and some time on Welsummers. The Barnie's I add are coming from a fairly local breeder and I'll be working with her to improve SQ and egg color.
Anyways, I hope that helps.
God Bless,