5 gal carboys mostly as they were leftovers from Mom and Dad's winemaking days. But I recently got a lid for my bottling bucket and must say that racking secondary couldn't be easier from that spigot (with hose attached of course). Planning on gettting a 6.5 gal carboy soon - for Christmas maybe??
Glass, glass, glass, I can't say it loud enough, GLASS! Plastic is evil!
If you use both plastic and glass and use plastic for your primary, you have your materials backwards. Primary fermentation is the most critical process in brewing. The beer is at it's most vulnerable and being able to observe what's going on is helpful (and really interesting to watch). Using glass for the primary is really the way to go, not plastic. If you must, use your plastic bucket as the secondary, but frankly, if you trust plastic (which I don't), you ought to just use plastic all the way around. That way, you get the perceived benefits of plastic for everything. Or, you can do what I did and ban plastic fermenters from the brewery altogether. Not that I have an opinion or anything.....
I've been using plastic buckets since the beginning & will probably stick with them. They are just too easy to work with and I haven't had a single problem tracable to the plastic. I don't have any desire to watch the yeasts working. I don't bottle much either.