Alright I've been looking at starts lately and have noticed that the Enclave MkI power armor has only 400 HP.

The T-51b has only 3 less DR, but 2000 HP...Wich one should I choose to just be a tank who kill everything?

Do you have the jury rigging perk? I haven't had trouble using metal armor to keep the Remnant Power armor in good repair.
If you do go for the T-51b make sure its not "Brotherhood T-51b" as that will give you the faction disguise.

There is a T-51b that doesn't give you faction standings and can be repaired with all those Brotherhood T-51b sets you'll find in your travels.

My advice is to get Cass as a companion and use her Whiskey Rose perk. You won't notice the DT difference but the durability will make you cream in your jeans.