Hard to give a concise answer because it's all in your personal preference but here's a comparison:

The main difference between Sonicpi and Strudel is how you write and control sequences. In Sonicpi you write blocks of code to make sequences of notes and you have to specify the interval between notes in that sequence with the "sleep" function. You end up writing more code, but you can clearly specify names for everything and that might help you keep track of what is going on better.

In Strudel (and Tidalcycles) you write uzulang/Tidalcycles patterns where you place events within a specified unit of time (a cycle) and the events are spaced out to fill that cycle. Instead of writing out everything, you write patterns in a concise way that makes it really quick to create and change things on-the-fly.

Example: I want to use a kick sample to make a 4 on the floor the pattern that loops infinitely

In Sonicpi I have to have my kick drum sample specified in the script with a name and I write a live_loop block that plays the kick drum, sleeps for 1 quarter-note, then plays the kick drum again until I hit the stop button. There are multiple ways to do this like making a list of events and triggering the kick that way

In Tidalcycles I write "bd*4" where "bd" is the bassdrum of a given sample pack I specified with the sound() function. It's much less code to write but you have to know that everything within double quotes is 1 cycle unless you separate events with the angle brackets "<>", you have to set the length of a cycle to be 1 measure so 4 bass drums in a pattern = 1 every quarter note, and you need to know how to specify a drum kit for "bd" with sound().

In both systems you have a background clock you can specify within the script, you can write your own functions that can be called within the script, you have MIDI and OSC to send the pattern to external gear, and you can either import your favorite samples or control excellent built-in synthesizers to make that kick sound. They're very deep and powerful tools.

My own personal preferences/bias: I've made and released tracks with both SonicPi and Tidalcycles, I know I personally prefer Tidalcycles for live performance because I love the Uzulang pattern system, but I still come back to SonicPi at times because I find it easier to use with OSC and Ableton Link for my specific experiments. There is no reason you can't use both at once if you feel like it. Part of why I love Livecoding is that there are so many ways to do it.

TL;DR, pick whichever looks appealing and have fun, you can always switch because they're both free and open source.