Hi imcintyre,
The best advice I can give you is to just jump in and try both wiki's and decide based on your own evaluation. There isn't alot of work installing them really. If you install the each wiki in its own ibay then you can experiment without screwing up your system. If in the future you decide to delete them, then you can simply delete the ibay.
Also, reading on the respective web sites of each brand of wiki will help you compare features. Looking into their user forums will also help you get an idea on any problems being experienced.
In terms of compatibility with SME6 or 7, you should look at what is required for each wiki. Some need a minimum version of python, perl or php. A quick way to check what versions of these SME has is to look at SME's distrowatch.com page
http://distrowatch.com/table.php?distribution=smeserver and look at the package versions.
You didn't elaborate as to what you intend to use the wiki for (just yourself, collaboration with others, public, etc). For securing remote access, you have a few options...
1. Allow by ip address or range on your firewall (this would restrict access to amything running on port 80 though)
2. Configure the ibay to be password protected
3. Use a port other than 80 but easy to remember, such as 8080 or 10080 (security through obscurity)
4. Make the wiki public, but use its own user management to control editing
5. Any combination of the above
Anyway, hope that helps. Good luck.
Regards,
Owen