A 20A/hr battery will run for 3 hours longer at one amp draw (in theory at least) than a 17A/h battery.
What's important for a start battery is the CCA value. Can the battery in question deliver the amperage at design voltage needed to start the engine reliably and repeatedly? And have some reserve for hard start situations? There are a whole boat load of other factors come into play. Charge level, ambient temps, oil, etc, etc. But the CCA number is a better value to consider for comparing starting batteries than A/hrs
The attraction of the Odysee is it's ability to survive deep discharge as well as operate near the norms of a conventional lead/acid setup. Most LA setups, if deeply discharged (left those lights on? Turned the key to far when parking again, eh? or like me - left all the radio gear running.) will degrade their ability to provide sufficient amperage at design voltage for starting. Do that enough times and you've a toxic paper weight. I've collected several.
I've been using a 17A/hr rated battery for better part of 3 years now - no issues at all. I've run it totally dead many severals of times too (those pesky radios!) and it still keeps on going. The 'traditional' LA battery I used to have "died" totally on me after the 3rd accidental deep discharge. That's when I started looking for an alternative.
Marine style batteries - like those used for trolling motors - are designed for deep discharge, but they don't supply the short durration, high amperage power needed for starting - well. They'll do it a few times, but it degrades the battery for deep discharge. Kind of the reverse of a traditional starter battery.
The Odysee is designed to do both - deep discharge AND high Amp starts - reasonably well. It doesn't do either as well as a single purpose battery might - but it does do both better than a either type of single purpose could.
So, if you're after maximum CCA or you're after maximum A/hr - each of those qualities are aimed at different duties. Choice is good!!
Me? I want my bike to start in spite of my in-attention to a couple extra switches and my laziness at not installing a 'cut-off' relay - yet. So far - it's done very well for me.