I think for us when it comes to 1200ft spools you simply cannot beat the genuine Elmo metal reels. we have many of these and when you spin them they are dead straight,no side wobble at all. Not a single film we have on these has ever had any problems, and when you get near the end of a reel during projection you never get that annoying scraping sound that you get with cheap plastic reels. You know the noise dont you, when your audience starts looking back to see what the noise is?
Having said that, The American reel company spools,(800 foot reels) are also superb. Perfectly moulded with no strain on the film.
All our 400 foot spools appear to be plastic and with a few exceptions we dont normally encounter any problems, i always throw away any home made ones which have appeared on a few second hand films over the years as these are always well dodgy and almost certainly puts a strain on film during projection.
If the reels dont wobble and the film flows out perfectly, the spool is good. Any side wobble or scrappy noises, get shot of it.