ReadyNAS Ultra 6 expansion

Hello all,

first off, hats off to Netgear for their products and to all of you for creating this amazingly resourceful forum. However, I have two specific questions regarding my setup I haven't been able to find here yet.

Here goes. My setup is ReadyNAS Ultra 6 in X-RAID2 mode with 6 x 2TB discs, giving me 9250GB of space. Probably redudant info, but I'm running on latest firmware and RAIDiator. I'm now at 99% capacity so I'd like to expand.

My first thought was to replace my six 2TB drives with six 3TB drives, but then I had the idea to get four 4TB drives. Costs are approx. the same.

Question 1: is there a good reason to choose either of the two options? What would you choose and why?

Question 2: going from 6 x 2TB to 6 x 3TB is easy. Take one disc out, put a new one in. Let the NAS resync, reboot, etc, and do the other drives the same way. Gonna take a while, but it's easy. What about the 4 x 4TB option though? After replacing the first four 2 TB drives with the four new 4TB drives, can I just pull out the last two 2TB drives safely?

Bonus question: I'm reading a lot about the 16TB capacity limit. Will I encounter this problem with either setup? I'm not willing/able to do a factory reset.

Hope everything is clear and you guys can help me out. if you need any additional info, please ask.

Just in case it's useful: my current drives are Samsung HD204UI's, the new ones will be WD RED's.

With many thanks,
Ruud