The oldest buses have lifts and are (slowly) being phased out.
All other buses have ramps.
The newer buses also kneel which makes the ramps easier.
The newest seem to have more and different wc/ECV space.
Under normal circumstances (other than peak times) buses tend to run on about a 20 minute headway. Buses are assigned their routes by a computer, although a dispatcher can override the computer. There really is no advantage to waiting for another bus.
I definitely would not wait for another bus if I didn't have to. You could be waiting a long time and the next bus could be the same bus as the first one was.
I agree with the others that there is no set rule about which bus will show up where or when.
Some people find the older buses with lifts more difficult to manouver their ECV onto, but most don't feel they need to avoid them.
There is more information about buses in the disABILITIES FAQs thread near the top of this board or you can follow the link in my signature to get there and then look at post #1 to see which post in that thread is about the buses.