Yeah I saw that Amazon link it's still in my wish list I wished I knew how much EXTRA run time I would get.. I don't think any of the prices HD has are that good..
Amp hours are amp hours.
The 4.0 battery will give you about 20-25% more run time than the 3.0 battery, depending on the tool and discharge rate. Apparently, some of the Milwaukee 18v tools perform differently based upon the battery used.
The larger battery also means that it will take longer to charge. Getting the fast charger may be a good idea.
One of the HD's in my area has the remaining 4.0 batteries on clearance for $65.00.
Note that there are apparent significant differences between the 4.0 and 5.0 batteries per product labeling (construction, internal electronics, venting, water intrusion protection and operating temperature range). The 5.0 battery may actually be a better buy.
There are 6.0 and 9.0 Ah batteries in the pipeline.