For Meraki switches, the different models generally coincide with features and port speed.
So which model you opt for generally depends on your architecture and need for various features.
MS12x are perfectly suitable for L2 switching with no frills.
MS2xx introduce physical stacking and routing
MS3xx and MS4xx move into higher bandwidth and multigig territory
https://meraki.cisco.com/product-collateral/ms-family-datasheet/?file
Within the models, you also need to look at the different amounts of power available for PoE+ output to connected AP's.
Generally speaking, your bottlenecks will always be the WAN connection from the MX rather than bandwidth across the switch, so a switch upgrade may not do anything to improve file transfers if they're going cross-site.