I have 6 bikes now, two with thumb shifters, one with bar-end shifters, one with down tube shifters and two with brifters. THe bar-end bike used to have cross-fire shifters.

I like the brifters since I ride in traffic and don't have to take my hands away from the brakes to shift. Also in the rain, with my rain cape covering the handle bars, I don't have to hunt for the shifters.

But I also like the simplicity of the down tube shifters and I feel the slight bend and reach reach down to shift, followed by the slight raising of my torso back to normal position adds a little extra core-strength maintenance.

Thumb shifters on the bars are my least favorite, but were cheap workable options for renovating my folding bikes.