I have nothing but Youtube clips, and stats to go by, but neither paint a very good picture for these two.
Sisto is 23. This should really be the start of his peak statistical years, but his scoring rate is very poor. 11 goals over the last 2 years. Bear in mind that La Liga is the easiest of the big leagues to score in. Absolute chumps like Aspas and Carlos Vela have scored there prolifcally. If Pisto can't score in Spain, he's unlikely to ever do so in England. I'll pass on him pretty easily.
Gelson Martins is more intriguing. His pace looks frightening, and he seems to have some skill as well. Yet the Portuguese league is a place to avoid buying from in general. Looking at his clips, there's an Aaron Lennon vibe you get immediately. No guile to complement his pace and skill. Very one-dimensional, lots of kicking and running into blind alleys, wild shots and crosses. I'd pass on him too.