I've used the 6V 4.5ah batteries and they work OK but don't last as long as a standard 6V without a solar panel.
With a solar panel, they will drop off during a couple of weeks of cloudy weather.
If your box has room, a pair of the 6V 7ah batteries work well.
I use these exclusively now and have enough for all my feeders + 2 extras.
I have a charger at the house and keep the 2 extras charged so I can do a swap out easily.
I usually got 1.5-2 years off the 4.5ah versions but the 7ah versions are 3 years old and still going.
It probably has to do with drawing the 4.5ah ones down below safe voltage whereas the bigger one has the extra amperage to handle load longer.
They are designed for security and UPS use so they are usually built better also.
Here is a shot of the 6V-4.5ah on left and 6V-7ah on right.