Foundation and Foundry are both great products. Because I work for Joe, I am of course biased towards Foundation. Here are my reasons why I think it is a better product.
It has a vast user base. Many of the sites on the RapidWeaver Community Gallery pages are made with Foundation. It has 115 likes on the Foundation page on the RapidWeaver Community page, you can see it here
It is based on a great Framework by Zurb, of course called Foundation. You can see information here
they have a showcase area and a brands area that show you what can be done with that Framework and which big name companies are using Foundation. Now, they are currently on Foundation 6 and Foundation for RapidWeaver is on their version 5, but that will change. One really cool thing about the Zurb connection, is Joe is constantly in contact with the developers of Zurb. He has been down to their headquarters in the San Francisco area many times, as recent as this month (December 2016). They help each other out with issues that either one finds with the other’s products. They are a great resource to have for a developer like Joe.
Third Party Developers. There are a ton of 3rd Party (or is it 4th or 5th party??) developers for Foundation. Many make Project Files and stacks that are for Foundation so you can get them and see how they work, here are just a few off the top of my head:
Big White Duck - http://bigwhiteduck.com - Makes stacks, some JUST for Foundation, others for any theme
Hipster Weaver - http://hipsterweaver.com - Makes great project files, some with Total CMS set up, so the hard work is done for you.
Jochen Abitz - https://jochenabitz.de/shop/?lang=EN - Makes great project files, again, some with Total CMS setup.
Webdeersign - http://webdeersign.com - Makes project files, not sure about all of them as I have not looked close enough at them.
I believe there are a couple more, but not sure of their websites or names.
Weaver’s Space - This is a great little community that Joe has set up, NOT STRICTLY for Foundation, but a lot of Foundation talk does go on there. The users there are a great help to each other.
If you are going to use Total CMS, there are MANY tie ins to Foundation that make using it even better than using it with any other theme. There are no differences between using Total CMS with Foundation and any other theme, but things like selecting colors, there is a choice to use what Foundation has set up, so you do not need to find that one color that you used for this or that, you can actually choose it.
Again, Foundry is a great product by a great developer. I would call Adam a very good friend, we have chatted many times online and I use to beta test for him on his older themes and stacks, even when I worked for Joe. Foundry is made by Bootstrap, another great Framework. Both themes/stacks sets should work with RapidCart Pro (I can not tell you for sure Foundry does, but I do not see any reason why it would not). Most other developer stacks will also work with both. There are so many stacks out there though, that I do not want to say they all will for sure work. Which ever one you choose, you will make a very professional looking site.
Again, I would say go with Foundation, but again, I am biased.
About your other questions:
If you need a calendar of any kind, DateSnap is a good stack. It can be used with Google Calendars too, so can be edited from outside of RapidWeaver.
As far as other stacks, it all depends on what you need. Would need a little more information before commenting.
EDIT - One thing I forgot to mention, that I think makes Foundation a little bit better than Foundry, and that is the fact that you can warehouse your images in Foundation. With the standard Foundry stacks, that is not possible. If you are not sure what warehousing is or why you should use it, check out this post by Will Woodgate, he has a free PDF explaining why it is a benefit.