The MR36 has two spatial streams per radio, whereas the MR42 has three. This means that in ideal situations with the 5GHz band for WiFi5 or lower clients the MR42 can either deliver more performance to one client or better server more multiple clients.
The MR36 supports WiFi6 whereas the MR42 does not. This means that in congested multiple user environments with WiFi6 clients the MR36 will deliver a better user experience and probably more throughput.
If your environment is congested (in the wireless sense) and you have all WiFi6 clients, go with the MR36, if you have mostly WiFi 5 clients the MR42 might be a bit better for now.
If your environment is not too congested (i.e. there aren't a large number of interfering Wireless networks within range), and you only have a smallish number of users and devices, say less than 30, I'd go with the MR36.
In our office we replaced MR34s with MR36s and even though most devices are WiFi5 it worked better, due to fairly low congestion.
One point to note, if you have users with Intel WiFi chips in their laptops and the drivers are over a year or so old, they may not support WiFi6, so they will not see the SSIDs! Every laptop we have had this issue with, has been able to be upgraded to work.
@cmr is correct, if your standard deployment is MR42, then the recommended replacement going forward is the MR46. It's one of those you-get-what-you-pay-for things, because a 4x4 AP has that 4th radio chain and will give better results than a 3x3 AP, all day, every day. There will not be a 3x3:3 WiFi6 AP, they will either be 2x2:2, 4x4:4 or 8x8:8. And depending on which and how many of your clients will be 11ax, open a case and work with Meraki Support to select the best options regarding disabling 11ax (for now) based on your deployment and what options you might be configuring per AP vs per SSID, etc.