How heavy are the round bales and the forks or spear you'll attach to the SSQA to lift them? Keep in mind that the lifting capacity spec is at the pins but the weight (center of gravity) of a round bale is farther out than that so the real lifting capacity is less. The BL25 loader's capacity is 2100 lbs at full height ( they dont' say but presumeably at the pins). If your bales are 2000lbs you're not getting them to full height. The 4015 loader's full height capacity is 1800 lbs.
The 20 series is larger and heavier than the 15 series. That makes is more suited to lifting heavy items.
For ground engaging PTO implements I think HST is better than gears. It lets you change your ground speed easily while keeping the implement running at the correct rpm. To do that on a gear tractor you need to shift, and you're limited to the ground speed that PTO rpm and the gear provide.
The 4015 has a turbo. I like turbo cars but it adds one more thing to go wrong.