David,

My personal opinion [and it is just that, and may not be worth $.02 ] is that the grain and color running vertical would be preferable to "rings" as you say, and the "2 face" might just add a lot of interest to the piece.

As for structural integrity, it seems that the weight of the piece would be better supported by vertical grain as well, [ think framing walls for a house, the vertical structure of the 2x4s are utilized]

The best effect? My opinion is vertical, but you do need more seasoned turners with hollow forms than me to give you information as to the "best effect"