Hi Annie,
The best one that I've used to date is called Power Article Rewriter. When I was first getting going I invested 3 times as much into an article re-writer and the complexity of it at the time was ridiculous.
This one in particular is very simple to use and the results are far better than the re-writers of the past. It's available to try for free for 15 days. After that I believe it's either $47 or $67. I can't quite remember, but either way it's well worth the investment for a good quality re-writer.
http://www.powerarticlerewriter.com/As far as your submissions all being different.
Usually a good rule of thumb is to always...always...make sure your best version is on your own sites or blogs.
Also, be sure to have unique versions at at least the top 5 article directories. i.e. Ezine Articles, Go Articles, SearchWarp and a few others.
If you are doing mass submission, be sure to submit to the top 5 by hand and do it properly.
Be extra careful to make sure your submission to Ezine Articles is completely a unique version. If they find out it's found other places on the internet they will disable your account. And since Ezine Articles is where you will get your most benefit you don't want to mess with that.
Back to mass submission services. I'd usually send about 5 different versions into mass submission services...try different categories if they suit them as well at times.
Since a lot of the directories that are used in mass distribution services won't generate a lot of traffic, but are helpful for SEO purposes with your anchor texts etc. it's certainly not as big of a concern.
But that said, this software will spit out so many versions don't worry, you'll never run out of different versions if you always wanted to mix them up.
Hope that helps...cheers
Davin