One unit is not necessarily better than the other. It really depends on what you are looking for. Each booster has its advantages.
The Sleek or Drive S units are cradle options, and will require you to place your phone in the cradle to receive the boosted signal. This will mean you will only be able to boost the signal to one device at a time, and you will need to use speaker phone, Bluetooth, or a wired headset to talk on the phone. However, since the phone is sitting directly on the indoor antenna, you don't experience the loss that you will with a Drive M or X booster.
With the Drive 3G-M and 4G-M, you are able to boost multiple devices, and do not have to keep your phone in contact with the indoor antenna or in a cradle. This booster has a higher gain than the S series or Sleek boosters, but you will experience loss through the air from the indoor antenna to the cellular device.
The Drive 3G-X and 4G-X have the same gains and advantages as the M boosters, plus they have a stronger signal headed back to the tower. They are able to boost the signal to multiple devices, don't require your device to be placed in contact with the indoor antenna, and are going to give you the best boost from your device back to the carrier tower for max performance in rural areas.