Knowing the SAC is the more valuable thing in my view. AI is a nice to have, because it's painless, can see it over a portion of the dive with the right tools (e.g., before & after that shark came by!), and it provides feedback DURING the dive.
However, if you take the time to record pressure a minute or so into the dive (after the initial cooling is over) and at the end, you can get the overall dive SAC easily without AI. It's a cost vs benefit thing that will be a personal call.
+1 on Subsurface. Can enter the start/end pressures manually so it knows your SAC if you don't pony up for AI. Seeing that decrease (it has graphs) as you gain more dives is... satisfying.
However, if you take the time to record pressure a minute or so into the dive (after the initial cooling is over) and at the end, you can get the overall dive SAC easily without AI. It's a cost vs benefit thing that will be a personal call.
+1 on Subsurface. Can enter the start/end pressures manually so it knows your SAC if you don't pony up for AI. Seeing that decrease (it has graphs) as you gain more dives is... satisfying.