I have been accused of not being able to purchase toilet paper without doing exhaustive web research. This made finding a quality trailer manufacture a futile task when we bought our trailer. Tons of research revealed that there is no such thing as a quality trailer manufacture. This means you should select a trailer that best suits your needs. Personally I prefer a shorter trailer with a slide-out. Shorter is easier to tow and fits into campsites easier. Slide-outs give more interior space when camping. Slid-outs do seem to eat up storage space through... there are always trade offs. You just need to figure which trade offs you are willing to make!
Buy your second trailer first is good advice.
Buying a used trailer as your first trailer is also good advice as you won't really know what you value in camper design until you have lived with it for a while. You will then have a much better idea of what is important to you.
The most important advice is "Don't believe anything the sales person tells you"! If they tell you "the sky is blue" I would seriously question that statement!