Yes, this is my favourite fish. But make sure you are getting the one with orange on it. The other one wont survive long.
I will help you Just2Bad.

How long do you have this fish ??

is it a good idea to add some paraguard when aclimatizing ??

Please try to rephrase this. My A. nigricans has now hit a point where it has survived 4 times longer than my A. japonicus did. It is growing, eating well and showing none of the regular signs of being dead.
What you probably meant to say was that the other fish (white cheeked) has a poor history of survival and/or requires a greater amount of TLC.
These points were one of the main foci of the article I linked to.

The article also said that " A. japonicus has a much larger white eye patch. Ranges from the Philippines to Japan and is relatively hardy."

The very similar A. japonicus is a far better aquarium fish; A. nigricans rarely lives for more than a few months in captivity.

So it should be easier to keep than a nigrican ????

Most people will tell you that the japonicus is the easier to keep. Almost all my research confirms this. My experience does not confirm this. But I can only compare 2 fish I have had, not the average. Dale D. from animal house states that neither are very easy to care for.
If you do a search on RC, you'll find lots of threads with experience keeping both of these fish. I think the secret is getting a healthy fish to begin with.
If you're planning a japonicus, I would stay with the plan. I just like to point out to some tang cops that no tang is "impossible" to keep. I have a nigricans because I was told it was the easier to keep based on Scott Michael's opinion, and so far it is doing well. I expect to report the same thing in the years to come.
Whichever you choose, make sure it is fat, healthy looking and eating well...good luck!!

They should receive two japonicus next week a medium and a large size, so I'll check them carefully before deceiding wich one I'll take and if they look too thin I'll pass on them...

I would prefer the smaller one myself, thinking that smaller equals younger. A younger fish may be more inclined to try new foods and adjust to the confines of a tank.

Brad is right and Brad is lucky.
I would not consider this fish though. In any event he japonicus is much nicer looking.
I have had the fish for seven years.
Do not buy the tangs unless you know where they came from.
They need to be alert, clear eyes, spot free and plump and eating. If not pass on the fish.
Get ready for it by arranging the following.
1. Buy some garlic extract from the health food store.
2. Buy some spirulina discs and unflavoured nori.
3. Buy some selconm
4 Have your temperature at 82F
5. Salinity at 1.025
6 Provide a clear area for the tang to swim from one end to the other unencumbered by coral or rock
7. Add at lease one more powerhead about nine inches from the top of the tank. This provides current for the tang to swim into and also adds more oxygen into the tank.
8. Do water change tonight.
Let me know if you need help

The only think I miss on your list is the garlic extract... I'll try to know tomorrow from where they're coming from.. I think it should not be philipine and/or japan ??

Garlic extract is very expensive. Here is what I did when I brought my tang home. Got fresh garlic and minced it as small as I could. Spread it on a piece of wax paper. This will make the oil puddle. Then wipe the oil up with some nori and feed. Worked like a charm. The active ingredient in garlic is suspeceted to be a compound called allicin. This breaks down quickly, and is why I choose fresh garlic. Thirty six hours after feeding, the spots were gone. I fed this for about 3 weeks.