On my first playthrough, I picked up Rex thinking "Oh cool, Dogmeat was the only companion I used in FO3, and now there's a robot version!" To my dismay, I discovered he dies quite quickly in even the easiest situations. Now my question is, if I do his quest and give him Lupa's brain for a +200 HP bonus, is he worth it? Or should I opt for the speed or attack implant instead and just guard him better?

Opinion/thoughts?

You could do a hard save and try all 3 types to see which one works better for you. I went with the attack upgrade for Rex, but only after trying out the other 2 brains first. Speed isn't too bad and the defense uprade is ok if you don't want to babysit him as much.

Is the attack upgrade very noticable?
And the same with the speed?

Speed will make him move around faster and allow him to deal out more attacks per enemy than normal.

Attack will allow him to take a bigger chunk out of bad guys, meaning he'll deal more damage per attack.

I know that but..which would be more bennificial?

I guess it depends on your playing style. I recommend the attack upgrade and listen for his growl. When he spots an enemy and begins his attack, work with him to drop enemies quicker. If you get surrounded by multiple enemies, try disarming them while Rex chews on their faces.

Another good thing about Rex is that you can have 1 other human companion aswell. I'm sure you already knew that, but if you travel with other companions besides just Rex, you could go with the speed. While you and your humanoid companion blasts/softens up the enemies, Rex will charge in to finish them, helping you save ammo or action points for the next unfortunate soul who crosses your path.

The defensive upgrade can be usefull if you don't want to have to watch over him constantly. It'll just be tougher too take him down depending on who/what he's fighting, giving you time to drop any other enemies who might be lurking around.

I have had Rex with me for over 40 hours of game time now. My choice was the attack brain and for me it is a very obvious choice when you see him in action. Rex and Boone is a pretty good combo for me.