I am using EOSRP for low light pictures, wanna make small prints 10x15cm or 13x18cm so resolution isn't an issue.
I can either use 17-55f2.8 in crop mode or 24-105f4 in full frame mode.
Knowing both ways pictures will be downsized before printing which option will give me cleaner files for 13x18 prints.
In terms of equivalence, your combo of a 17-55 f2.8 mounted on a Canon APS-C sensor is equivalent to a 27.2-88 f4.5 lens mounted on a Canon FF sensor.
So in terms of total light gathering ability, the 24-105 f4 on your RP is better.
This is of course assuming both sensors use similar tech. I know the sensor in the RP is pretty old tech, but I don't know what APS-C you're using.
The f4 lens is a tad "faster" than the f2.8 APS-C lens, in equivalence terms. The f2.8 crop mode will give less noise per pixel, but the FF f4 combo will scale down the noise more. All in all, the FF f4 version would be a tad cleaner, because of the tiny bit faster equivalent lens, where both noise issues cancel each other out.
Many thanks for your help, tried both and, if there's any difference, it's not noticeable at least for me.
So I opted for 17-55 in such situations for the files are smaller and faster to transfer and process
I just realized I read your first message too quickly and I didn't reply to your question :-(
BC answer is correct.