DMS and CTI are equally simple, with the main difference being you drop the CTI reload into a case and screw it shut. They are both very easy compared to building an RMS motor, and that’s why these are all I use — I like it simple and don’t like “building” motors, so I don’t even own any RMS hardware.
With CTI, you would need to buy the case as well as the motor, but sometimes there is a cert special that gives you a discount or even a free case when you buy the motor for your cert flight. I used a DMS H115DM for my cert flight, which was super easy and a great flight. Later I found out I could have got a free CTI case if I had gone with CTI. For L2, I got my 5 grain 38mm case for free. I’m not sure if there are any cert specials currently, but it’s worth checking. Good luck on your cert flight!