Hi, I'm making a cake for my grandfather's 75th birthday this weekend. It will be a fishing theme, and I'd like to make a rowboat and the fisherman from either MMF or candy clay. I've never had experience with candy clay, and only minimal with fondant. Which is best and easiest to mold figures and such with? Is there anything special I should know about working with either? Thanks!

I have never used candy clay for molding figures and such, only for flowers so I am no help there. I would tend to use chocolate molds and candy melts for some items.
Marshmallow fondant tends to be better to work with on its own than regular rolled fondant as it tends to set up a bit better. Usually though, I add a gum hardener like Gum-tex to anything I use for figures and such even if I am using a regular rolled fondant that has a bit of a hardening agent in it like the Wilton product does.
Actually in my experience, the best thing to use is gumpaste because it dries faster and harder than any kind of fondant will and it is more of a permanent thing, something you can keep for years. It is also much more strong. It dries and sets up much like plaster or clay items do.
For larger items like figures, when using fondant, I use support inside, perhaps a wooden dowel or toothpicks, wooden skewers etc. to attach heads, legs etc. I also use royal glue, a mixture using a ratio of 1 to 1, so 1 tsp. of water to 1 tsp. meringue powder to adhere things. Or you can make a glue mixing some water into a small amount of gumpaste or fondant. But for anything larger that is not thin or flat, simply wetting it with water is not enough of a gluing bond to keep it together. For some items royal icing is a useful glue also.
If you make larger or thicker items from fondant, they will set eventually but not really harden all the way through and will be effected by the grease in buttercream or by humidity so that has a bearing too on how far in advance you make them and how far in advance they sit on a buttercream iced cake. Thinner items will absorb the grease from buttercream and break down faster. Generally, the longer drying time they have, the better.
I allow at least a week's drying time for most items when making them with fondant. Sometimes I use the ratio of 1/3 prepared gumpaste to 2/3 prepared fondant for some items. Otherwise I mix about 2-3 tsp. Gum-tex powder to about a golf ball sized piece of fondant.
Another thing you can use as your hardening agent is the powdered form of Fix-o-dent, thanks to Sewsweet2 for that information. It is food safe.
Since generally these decorative items are not eaten, there is no issue with them being hardened.
Hope that helps.
Hugs Squirrelly

personally I like working with candy melts mixed with corn syrup its like playdoh and firm enough to make really nice detail. the recipe is on CC site.

I much perfer candy clay...I hate to color mmf, I break my hand kneeding it...I do the microwave trick, but I still like candy clay better.


But as far as taste I like a rolled butter cream. I bit greaser than mmf. but tastes way better. You can do 50%50 with rolled BC and mmf, that works well too

Good luck