Glass is always better than plastic for fermenting your beer. Plastic scratches very easily and is more difficult to clean. Go for glass carboys if you can. Fermentation is the most important part of brewing your beer, so you don't want to go cheap here.
Also a yeast starter kit is something to look into. You need to pitch enough yeast to make good beer (if you don't pitch enough you're beer will suffer). You can do this by either buying 2+ packs or vials of yeast for $7 a piece, or you can make more yeast for the cost of one pack with a yeast starter. Worth at least looking into and saving you some cash in the long run (with that said though, a yeast starter can be made cheaply with a gallon glass jug, but a starter kit will make things a bit easier).