I did another playthrough this time on my iPhone 4. I died sooner. Not to say it is more difficult. It may be easier on the iPhone to control.
The controls on the iPhone seemed more optimized and snappier than when I played it on my iPad.
But Screen Real Estate was an issue. It's worth noting you can make the screen a little bigger by going into the options and selecting large.
When controlling the character my thumb initially covered a good deal of the screen. So I started using my index finger, this seemed to help but wasn't quite as natural as my thumb. Making my finger footprint as small as possible was important so I don't miss monsters. I even experimented with using my pinky! This was effective but too difficult to be useful.I found the best way to control was at an angle using mostly my fingernail to actually control it.
Overall, I am a believer in touch controls for shooters and almost find it prefable to a beautiful Sanwa/Seinmitsu 8 way joystick.
I think it's possible touch is the future and we may even see actual cabs using highly sensitive touch panels. (Imagine having a dual screen setup, little touch screen with your game so you can tap areas but blasted on a monitor. I think the potential for highly precise and potentially highly skilled gameplay is there. And could actually help the arcade market.