I would recommend guppies because my platy just died -.- Lol, really, I like platies and mollies because they are active and fun to watch, but guppies look really nice ^^ It's up to you
I'm not sure if there's enough room in a 10g to get 3 - 4 platies (1M/2-3F) AND 3 - 4 guppies (1M/2-3F). If you are not interested in breeding, you could get all males or all females tank, hence you could get less of each and mix the two species.
That rule is over-simplifying the problem underneath. In general, fat fishes produce more waste than the thin ones, even if they are of the same length. Also, it is the surface area of the water (that meets the air) that determines the stocking capacity rather than the volume.
At the end, it certainly is feasible to keep 3-4 platies and 3-4 guppies in a 10g tank, but this will become relatively high maintenance tank, requiring more water changes and perhaps more surface disturbance for better gas exchanges.
Having said that, I do have 7 adult platies in my 10g. It is driven by Aqua Clear Mini and a sponge filter, plus it is 2/3 full of java moss. I never lost fish in that tank, and I "only" do 30% water change once a week...
I've had guppies in the past, and I currently have platies. I've found I like the platies best -- they have a lot of personality and they're very active. I have 11 fry right now too, and they're just darling. I never had much luck with guppies, for whatever reason. But in the end, its just whatever YOU like best!