If you just finished OA and could just take one of them, wich do you prefer?

Neither. T-51b looks good on Charon. That's about it. I do use the CSS for reverse-pickpocketing armor on buddies and townfolk.
Otherwise I roll in leather armor. CSS is too OP. It's a gamebreaker imo.

Interesting. Why is CSS so OP? Nobody can see you? Even when you shoot?

With a silent weapon(say the http://fallout.wikia.com/wiki/Perforator), yeah no one can see you.

In addition to Post 4 ^^: If my sneak is at 75, and I'm wearing this...
I can sneak around enemies during the day mind you, and they won't see me. If I stand up, they see me, then I crouch down and move around again.... and then in two seconds they don't see me anymore. So it's great for getting armor on your caravans, but the game is boring, at least for me, to wear this for more than what I described above, because, well, no one sees me.... ever, unless I want them to or I screw up a pickpocket.

If you invest in sneak the CSS is really OP.. If it is over 80 you might just well walk and bumb into creatures, people, ghouls... etc.. no one will notice you... it's like you're a ghost.
I was glad they didn't include that version in FO:NV...
On one PT I set my self the challenge to reverse pick pocket grenades on baddies an no matter what armor on other NPC's and voila near 80% succesrate.
The T51 looks really clean.. but is not much better than normal power armor (yeh DT wise it is better... but there is the wg issue)
Unfortunately the best armor of that DLC, IMHO, is not "normally" obtainable and that is the regular winterized combat armor. But even that is fastly outdone by better armor.(Unobtainable unless you say hi to garry 23.. or console it in)
So if you're question is what is the best armor since you can only take one..

1. CSS: Pro's : It's like having a continuous (though less powerful) stealth boy. If you play a sneaky assasin type and want to make the game a breeze... invest in stealth take a silenced weapon and have "fun". It's light (20 pnds)
Con's It can only be repaired by NPC's or by the relative rare Recon Armor.. It's DR can degrade really fast if you don't use it's stealth abilities..or are discovered...
2. T51:Pro's Depending on taste: It is the cleanest looking armor one can get without exploits. In a normal PT it is probably one of the fastest ways to get power armor and power armor training. and.. thus it is also one of the earliest oppurtunities to get into armor with a high DR. Offers (very little) rad resistance
Con's: Heavy, can lead to glitches with a sidequest in the vanilla game.. Stat bonus is mweehh... needs other power armor to be repaired or an NPC to repair it. So repair is either expensive, or have to wait for mid/late game to get a lot of repair opportunities..

For me I usually don't pick up either.. (well I do store them but don't use them.. only real reason to invest is Gauss Gun IMHO).. and wear either light or medium armor..
(Btw: for the love of god: Why people who play on hardcoe in NV insist on wearing PA and then complain about their encumbrance.. is beyond me..aaarghh...)

Chinese Stealth Suit! Both of my characters own it and wear it, until I get Reilly's Ranger Armor! CSS saved my butt a few times. I especially love wearing it in Ghoul invested areas. Those things creep me out big time! LOL