They're very different games and hard to compare. But if you could only play each one once, I'd say Lab Rats was the better of the two.
The first benefited greatly from its much tighter scope which meant that updates were entirely story and dialogue related (with supported imagery). Big additions to the game therefore came around on a very regular basis.
The second has (or had?

)
much greater potential and depth, but story and dialogue came at the expense of refining the 'game' side of things which seems to be Vren's main interest. It was looking like the mechanics were finally at the point of being mostly complete, then the Live2D implementation stalled things. If Vren is still working on this, then hopefully the Live2D challenge can be overcome, or abandoned, and the focus changes to story.
Lab Rats is a fairly short, fun and fappable game but doesn't have a huge amount of replayability.
Lab Rats 2 is one to come back to time and time again, and hope that it continues to evolve. Honestly the mod scene is what has elevated it from being okay to very playable so hopefully that continues if Vren does not return.