The Legend octo is a balanced 2nd stage, the Calypso/Titan is an unbalance 2nd stage. Whether this matters is debatable as there are companies like Mares who's 2nd stages have predominantly been unbalanced designs.
Amazon lists the Legend octo at $245 and the Calypso/Titan octo at $185 (which is ridiculous as I bought an entire Calypso reg set new at a local dive shop a couple of years ago for 145 euros).
The biggest benefit you will find with the Legend octo compared to the Calypso/Titan octo is if or when you do decide to sell it...the Legend octo will theoretically fetch a higher price on the used market than the other.
If that is not an issue for you, and your plans are to dive within recreational limits then you can certainly save money by going with the Calypso/Titan.
All that said, I have 2 Calypso DIN reg sets...the one I mention above and another that I purchased as just a 1st and 2nd stage to use on my pony bottle. I picked up a Core supreme 2nd stage for when my daughters are home from university so I could convert that pony bottle reg to a full reg set with the Calypso 2nd as the octo.
Also, I dive year round in cold dark lakes and quarries and will often switch to the pony bottle towards the end of the dive to keep proficient in switching to it and restowing it while maintaining proper trim and to ensure everything works well. I have yet to have a problem with either the 1st or 2nd stage...and the octo is just a yellow version of the primary calypso 2nd (the black one).
-Z