Unique content is the #1 method to rank well in search results, all article sites do is provide a backlink to your site. Your better off always keeping content for your site and re-writing a small summary of the article for article sites, then linking back to the original article itelf
I tend to submitt first purely because ezine will rank the article higher and faster. So I get the link out there for now and then rewrite the article for my site at a later date.
make a good article for your site at the first place, then make a sort of teaser but informative one for the zines
Placing article in your website and in the different article directory sites. This will enhance your traffic.
You should reserve the high quality content with important keywords at your site and submit others which you think may less superior. I can't tell you exactly which one is better or what criteria you can use but basically use this common sense. Only the best are reserved at your site.