I am really loving my Mainland Concert Mango, so any Mainland gets my vote. I don't own a Lanikai, so I can't really weigh in on it's sound or quality.
I understand your attraction to the Spalted Lanikai. The tricky thing there is that with spalted grains, each uke will look wildly different from one another. Make sure you get to see a picture of the exact uke you are buying. The range of coloring and grain pattern can be startling.
As for size, tenor is definitely closer to guitar in playability and sound. You will have an easier time on the left hand with more room, but you will also get a more guitar-like sound. If you want a more uke-like sound, go for the concert. I find the concert neck pretty comfortable, and I came from the guitar too. Keep trying on the soprano, it will get easier with time (it did for me anyway).
Christmas is coming, maybe you can get both....
–Lori
