Expanding my Synology DS1813+ unit

I am slowly running out of space on my DS1813+ unit that has 8x4tb drives in raid 6 (I use enterprise grade WD RE drives). I have 2 possibilities using the DX513+ expansion unit and I was wondering what you guys would recommend.

1. I expand my current raid 6 array to the expansion unit, this will make it 13x4tb, total usable space is 44tb. However, I have two concerns. The first concern is what if the expansion unit loses power or loses connection to the main unit? Will the entire array be corrupted/lost? Also, is one big raid 6 array too large in terms of URE failure when rebuilding? I have enterprise drives so the risk is much lower than consumer drives.

2. I create a new volume on the expansion unit, this would make it 8x4tb + 5x6tb, total space is 24tb for array 1 and 18tb for array 2. This is safer, but slightly annoying having two arrays instead of one, I lose space, and it costs more (6tb enterprise drives aren't cheap).

I'd prefer the first option, but I would be more comfortable knowing that I won't lose all my data if the connection was cut between the two units. Le'ts say the I do lose the array do to the expansion unit dropping out, can Synology rebuild it or is it lost forever?