I've found that when Spectro is held in the off season on a weeknight, it is very crowded. Not only for Spectro, but the entire day at MK. Go on Saturday night for the late showing.
I think you will be ok with the Tuesday show - we always view from the bridge near Splash Mountain and get a good view + it takes some time for the parade to get there so you can get a couple of Splash Mtn or BTMRR rides in with little or no lines
The Saturday showing sounds too late in the evening for me
I'd love to watch the parade at 10pm but the little ones would never make the late showing.
There is something special for me to see the parade coming down main street. Just what I love and to do it without having to wait very long would be heavenly.
I would go for the 6:30pm parade on Tuesday then you could always go Saturday if it is over crowded or if the weather is bad. (You will have already seen it if the weather is bad Saturday) I try to be flexible that way you'll have more options...
Well as lucky as I was on my past trip at finding a seat on main street for the late parade on saturday, I saw go for it. It was alot less crowded and it's just nice to enjoy the parade without the crowd.