I’m not a big water fed pole guy but if I was in your position I would definitely go with a water fed pole. Most likely if customers are asking you for window cleaning their expectations are not extremely high or they would call a window cleaning company. That’s not an insult but if my expectations were high and I need something pressure washed I wouldn’t ask a window cleaner to do it. That’s an advantage I would leverage.
With a water fed pole there are definitely situations where it’s not going to look as good as using a ladder and a squeegee but for your needs a water fed pole would be plug and play and make money. It’s a no brainer.
On the other hand if your wanting to add a professional window cleaning service and not just as an add on service then you would have to learn both. There are times a squeegee is necessary and times a who is most beneficial.
Just my take.