Unless you are going to load your own shells for the .410, I would recommend a 20 ga. 410 shells are very expensive and 28 ga shells are hard to find. You can get 20ga and 12 ga dove loads very cheap anywhere and can buy 2 boxes cheaper than 1 box of 410 shells.
I bought my daughter a 410 about 6 years ago and it has only had about 2 boxes shot through it. The 20 ga is another story, several boxes have been through it. The 20 can be used for everything from rabbits, squirrels, quial, doves, grouse, pheasants to turkeys and deer. The 410 will leave you wanting for more gun IMHO.
The LOP on my daughters Pardner is exactly the same as the 870 sitting next to it. My Ultra Slug Hunters LOP is actually about 1/4" longer than the 870.