Basicly I have the same question... using nikon D70 in ikelite too and did not want to start a new topic.
I am buying 10-20mm sigma and one of the FE primes/15mm or 10.5/. I would like to use the sigma for landscapes too, so it is a priority at this time. I have never had something as wide as 10.5 even on 1.6 crop... Although, I like 10.5 WA shots a lot, I fell in love with some of Alex's WA photos, I have to ask if it is a right choice for me. I mostly dive in the red sea, where I always count on some schools and wall shots. This is where 10.5 shines. But in case I can not get close enough to the school, the school is not big enough, or meet a shark, the 10.5mm does not give me the right angle.
The 10-20 is not that wide, but would be good choice for wrecks and can deal with wide angle scenery too. Wide enough for surface. No question about this.
Finally, do I need the 15mm FE? Would you, experienced WA shooters buy a 15mm when you already have 12-24 /10-20/ and a 10.5mm? Is the advantage of sharp corners more important than the versatility of a 12-24 zoom?
Shortly, which is a must?
thank you