In the end, unless you are doing nonstop big production work, you will find the speed pretty comparable. As in WAY faster than what you have now.
It's more of a usage question. If you are going to take the laptop to meetings and actually use it with clients then get it. It will drive up to a 24" monitor at 1920x1200 with no problems. If it is going to sit on your desk 99% of the time then you can just pick up and move your iMac the other 1% of the time then go with the iMac and get the better hard drive etc. for less.
FYI I have an old G4 Powerbook that I used as my desktop with an attached monitor until I bought an iMac last year. I still take the Laptop to meetings once or twice a month and can live with it being slower for those events. The clients don't notice. I use my Core2 iMac in the office and move it to the conference Room for those meetings it is very easy to do especially if you get the wireless keyboard and mouse. It has worked well. For everyday work I like the 20" & 24" monitor combination.
So what's it mean? Either way is a good way to go just try to be realistic about your usage and buy that way. A MacBook that just sits on your desk all the time is a waste of money. A MacBook that wows clients in the field is worth the extra money by the second or third meeting. But how will you really work?
Mac OS12.6 AC26 USA Silicon
M1 Macbook Pro