Blimey. I was going to ask this exact question in here.
I've found the same ARF DSSD on Puretime with two different movements (A2824 and SH3135) and as it's my first rep, people seem to be saying go for the A2824 as it's a workhorse movement and easily and more cheaply fixable and ideal for a first rep. It's also slightly cheaper too.
I have learned that the A2824 is the A2836 without the day function and the SH3135 is more 'gen like' with regards to setting time and crown position.
The thing is, the ARF DSSD on Puretime lists the same external dimensions for the watch for both mechanisms, and they also use the same photos. Is this them cutting corners for ease, or do the two mechanisms share identical watch dimensions (I understand the differences in movement side inside)?