Something I noticed that is interesting is the 13" models run 2133MHz LPDDR3 while the 15" models run 2400MHz DDR4. Both are 8th generation Intel's. Does that mean they run different chipsets?
This was a necessary tradeoff in order to offer 32GB RAM in the 15" machines. After years of delays, Intel's chipsets still do not support LPDDR4. It's possible that it's still the same chipset on both machines, though.
So the 2400MHz DDR4 is obviously faster because of the increased bus speed right? LP just mean low power.
That depends on the clock cycle latency but ceteris paribus yes, a higher frequency at the same timings would have higher bandwidth.
Even in benchmarks, I have generally seen very little performance gain with slightly faster memory.
The 15" sports a bigger battery than the 2017 model too. Just to compensate for DDR versus LPDDR.