Help me decide!

Solution
Those cut outs are overrated IMO. It's better to install the cooler with the motherboard outside the case even with plastic push pins so you can make sure they went all the way in (and have more room to access them). Also, I had a bad experience with its predecessor, the Cooler Master Hyper TX2 when trying to install it with the motherboard still mounted in the case. I've been told that it could have been partly due to the different leverage a tower-style cooler puts on the push pins.
Between those two, I'd expect the Cooler Master Hyper TX3 to perform better. The Silverstone SST looks like a low-profile cooler for small form factor builds, probably not really designed to give maximum performance. However, I'm curious: what case do you have and why is a slightly bigger cooler like a Cryorig H7 not an option for you?
 
Those cut outs are overrated IMO. It's better to install the cooler with the motherboard outside the case even with plastic push pins so you can make sure they went all the way in (and have more room to access them). Also, I had a bad experience with its predecessor, the Cooler Master Hyper TX2 when trying to install it with the motherboard still mounted in the case. I've been told that it could have been partly due to the different leverage a tower-style cooler puts on the push pins.
 
Solution