I play a David Webber bouzouki, which I like pretty well. But, if I had it all to do over again, and if I had a big assortment of instruments to choose from, I’d have chosen an octave mando I think.
The zook’s neck is just a little too long for my tastes (and I’m 6 feet tall fwiw). On some tunes I like to do some melody playing, and it isn’t particularly easy given the length of the neck.
So, if you plan to do any melody playing, that might push you toward the octave mando.
As for the issue of tuning in unison or in octaves, I don’t like how my Webber zook sounds when the low strings are tuned in octaves. That could represent a mistake on my part – maybe I didn’t choose an ideal gauge of string? But, I suspect that it isn’t just my preference, because when I got the zook (new) it came strung in unison. I think this zook just doesn’t sound as good tuned in octaves. A friend plays a Fylde zook, and I think it sounds great tuned in octaves.
I wonder sometimes whether there’s a rule of thumb here… maybe tuning in octaves sounds better on arch-top instruments? (The Weber is a flat top.)
I too think that GDAD sounds much better for backup for ITM. Given the common keys, having the top string be a D means you can let it ring open which is convenient and easy, and it sounds good.
My main instrument, before zook, was mandolin, so I was of course very familiar with GDAE. But as soon as I took up zook I found GDAD was better (for ITM backup).
I think the only reason to consider GDAE, for either zook or octave mando, is if you’re going to do a lot of melody playing, and so you want to make the higher notes an easier reach.
I have an octave mando on order, custom built. When I get it I plan to tune it GDAD, since I do much more rhythm playing than melody playing.
Even if you tune an octave mando to GDAE, still, I don’t know how much benefit you’ll get from mandolin lessons. Octave mando is a LOT bigger. A chord that feels comfortable on a mando will be quite a stretch on an octave mando.