If I could do it again, I would pick the clip on one - put it above your diffuser to find your 30ppm once you have your BPS down - relocate the drop-checker to furthest point of diffuser to see if your getting sufficient amount of Co2. I could see tweaking your lily's to get the circulation of the tank alot better with the clip-on compared to the suction cup.
The suction cup does stick out a bit inside the tank (my betta likes to lay on it) but it's kind-of a bugger to clean just because of popping off that suction-cup. Also cleaning around it is kind-of a pain too.
If you got glass lily's and a nice looking diffuser, definitely pick up the clip-on (just more sexy in my honest opinion)