ReadyNAS Ultra 6 plus vs. Pro 6

Well with hard drive prices so high at the moment that does push the budget for many. Funds are not unlimited. Before the floods I'd been hoping for drive prices to fall further before buying more ReadyNAS units. Now I'm waiting for prices to come back down. Even upgrading existing units is expensive. Would need to spend about $800 AUD to add 4x3TB drives to my Ultra 6 to expand it. Also as my SeaGate 1.5TB drives (not a very reliable drive, I know, but I've only had one failure so far and that was in a NV+ unit) are not supported in the Pro 6 I can't simply buy a Pro 6 and move the drives across. I'd need to buy a new set of drives.

Edit: That's interesting. Looks like my drives are now listed on the compatibility list for the Pro 6. Maybe I just missed that and hadn't noticed it before or noticed it and forgot about it.