Memory speed vs. amount
There is a good bit of "it depends" here due to the question being asked about both size and speed. I'm going to take the size out of it, either you have enough memory or you don't.

Talking just speed from now on and assuming DDR3 for the memory type.

The difference in transfer amount (theoretical) between the two is as follows:


DDR3-1333: 667MHz * 2 bits/clock (rise and fall) * 64-bit wide channel = 10.67 GB/Sec
DDR3-1866: 933 * 2 * 64 = 14.93 GB/Sec


This means there is (theoretically) 14.93-10.67 = 4.26 GB/Sec difference. Unfortunately that's just the memory part.

Continuing our way down the pipeline, we're going to hit our memory controller, of which almost all are integrated into the processor. Thus we'll have to look at the processor (and motherboard layout) to see the number of possible memory channels and throughput of the controller.

You can continue to do this for your specific hardware and figure out the theoretical difference.

Since you've specifically asked about SQL Server In Memory technology, having the faster memory will almost-always be better. That's not the whole case, though, since the way IMOLTP works is by using a specific assembly instruction (cmpxchg16b), thus having faster memory may or may not help if the bottleneck is the CPU speed (think 2.1 GHz [high density VM cpu] vs 3.2 GHz [low density high throughput cpu]).

Assuming the same processor and that the CPU is not a bottleneck, there is dual or triple channel memory, all memory slots are balanced, etc., etc., then yes having the faster memory would generally be better.