I'm pretty much split, as I find myself playing both series time and time again.
R-Type had a laid back scrolling approach, but the original Gradius wasn't exactly speedy either (shorter stages though, Gradius 2's Crystal Labyrinth stage is so damned long), R-Type had some pretty inventive and screwed up ideas for bosses ( which was actually cool). R-Type 2 played similarly to R-Type 1, while Gradius 2 got rid of the 1 boss for 6 stages approach that Gradius 1 had, I hold both in high regards.
Gradius 3 is revered for it's brutal difficulty (though tviks humbled it) R-Type 3 again took the weird boss approach, but the stage ideas were pretty nice.
Gradius 4 while difficult (not as bad as 3's difficulty though) was a good game in it's own right, I enjoy playing it time and time again, though it's tried and true formula never changed. Leo on the other hand got rid of the bits, yet still managed to be an outstanding game.
Gradius Gaiden was pure awesomeness and it's novelty with me still hasn't worn off, even if again it's tried and true formula never changed. R-Type Delta was the series first jump into 3-D, it's formula as well stay tried and true, though it's last stage threw me for a loop (baby sperm dead ahead captain).
Gradius V felt a little too mechanical, but it's length meant the game wouldn't end too quickly, at first I found the storyline strange but once I reached the end I understood it better. R-Type Final moved very slowly, but I found the boss fights pretty epic, the 100 ships to unlock gives it lots of replay value.
I'll give these two series an equal split and if stuck on a desert island with my choice of 3 shmup series to take along with me, it would be Gradius, R-Type, and DoDonPachi.
