Let me just start off by saying I own no Slizers or Roboriders and that I own many HF sets (all 1.0 heroes, most 1.0 villians, all 2.0 heroes, some 3.0 heroes, Witch Doctor, all breakout heros, and some breakout villians), so that "nostalgia" thing that many seem to have for picking a certain theme over the other, I dont have. I prefer both themes very much, however to me, I would pick Slizers and Roboriders over Hero Factory any day.
However I do agree that HF has a better storyline (altho I dont like it) because the line has been around for quite some time, so I guess thats expected.
Set design-wise, you cant really compare the two because they are very different from one another, even if they all came from the same root theme, which was Technic. Slizers and Roboriders being the ones closest to Technic would win to me, in my own opinion mainly because I like the Technic-feel of sets. I myself am not a fan of the HF ball and socket system, being that I consider it being too simple and I think it strives too far away from the Technic theme. I enjoy the Technic very much.
Now I know Aanchir, that yes some HF sets do have complexity to them (Witch Doctor, Drop Ship, Furno Bike, Von Nebula, etc.), however that isnt a lot considering that HF has 80 something sets released and almost all of them are simple designed sets to appeal to a younger audience.
The main reason why I like Slizers and Roboriders and prefer them over HF because I feel like I would have a lot more fun building them than building HF. But thats just me
Btw, Witch Doctor and the 2010 villians are the only HF sets I have that have not been disassembled and put into a bin.