I beg to differ on saying they're both "one of the best Sentai series". anyway...
both Jan and Takeru have good background story reveals: how we found out about Jan's true origins, as well as Takeru being a kagemusha.
in terms of character development, both series are pretty much the same with Red getting all the development he ever gets.
in terms of storyline, well, Gekiranger wins since their plot twist with Long being the Big Bad all along was more mind blowing than revealing Kaoru as the true Shiba Lord.
in terms of action, of course Gekiranger wins because c'mon, all Shinkenger did was just swaying their swords with style.
I actually don't like neither one, but since I have to compare, I'd say Gekiranger is the better series.