A few thoughts -- LATCH does not confer any safety advantage. It is a convenience. I have a LATCHed booster and I like it, but I wouldn't hesitate to use an unLATCHed booster (in fact, I do in husband's car) if it was a good fit.
"Side impact protection" is a term developed in marketing departments, not engineering departments. There is no standard for "side impact protection". Any seat can claim it regardless of what it offers, and some seats that do offer decent protection don't bother with the terminology. Look at the seats. Look for good deep headwings with energy-absorbing foam.
And most importantly, look for a seat that fits your child well. In another post you write that your son is 43#. The Monterey, while an excellent seat, does not usually fit small booster riders well, and IMO/IME, a 43# child is a "small booster rider".