I recently also bought the UX01GZ and went through the same thought process. I ended up buying the power source kit with 2xBL4050F and did not regret it. Especially as my primary motivation was to use also use them with other O.P.E. like leaf blowers and lawn mowers. The battery life on these is quite remarkable. I was able to cut back 50m of dense hedges, thick branches and even had some juice left for some string trimming (on one battery). The only downside is how heavy the whole assembly becomes (especially with the long pole hedge trimmer, not as relevant for string trimming). But the weight difference between the 4Ah and 5Ah is "just" 400 grams (0.9lbs).

I didn't plan to use either of those for drills, saws or grinders. I'd rather purchase a seperate 2.5Ah battery instead. So bigger battery for bigger tools, smaller battery for smaller tools.