I know I'm in the minority, but the Hammerhead is better. It's only prob was not enough levels, but at least it's levels were designed with actual gameplay and puzzling in mind. The Mako had no gamey element at all and the worlds were boring... a damn grind. Getting excited about it is like me getting excited about "Driving".
Maybe that was the case when I was 13, but not now.
I dunno if I could say that the Mako was really a grind, mainly because there just wasn't enough stuff to actually grind through most of the time. I did hate the difference in XP gain in and out of the vehicle though. I was glad that ME2 and 3 got rid of points per baddie.
As for the Hammerhead, I would be lying if I said that I didn't sort of wish that Shepard could say "Good riddance" when Cortez told her about the tech lab where it was kept being destroyed. I love the idea of a hovercraft tank, but it just fell so short for me. I think much of it had to do with the maps we used for them, but then map design was something I felt ME2 got very wrong throughout.
I didn't really have a hard time with the Mako myself, but then I put in an ungodly amount of hours in the ME trilogy when I first got my hands on it. I roved about until driving that bouncy jalopy felt second nature. But I guess also I just really love a good ground assault vehicle. It's just too bad that the stupid XP thing forced you to get out to shoot enemies when you're trying to get to that illusive level 60. I had a grander time in NG+ though.