Dear Erick,
I think you’re putting too much into it. Unless you’re installing it yourself. If the company that you’re working with is everything from design, permitting to install. Trust their judgement. Most reputable installers do a good job designing the best system for your need and no need to spend more that what you should. The difference is not tangible. Remember, the installer and their history and the type of warranty they offer is the most important thing for you to research. A lot of companies claim that they offer the best warranty but, it in the writing that matters. Also, if they’ve only been in business just a couple of years, their warranty is only as good as their promise.
In addition, make sure that the 21 panels (6.3 kw system) is enough to offset your usage at least by 105%.