Much easier from the bottom, especially if it is time to clean out the belly pan to prevent fires. I am not real good at standing on my head.
Some new aftermarket pucks have incomplete or distorted threading in the mounting holes. I would suggest that you run the mounting bolts completely into the new threads on a bench, rather than find out while working underneath the machine. Adjusting the engine alignment is critical after replacing the disc. Uniform adjustment as per your manual, from the flywheel to trans flange is also easier accomplished from the bottom.
It is also a good time to replace that trans flange seal that is probably already leaking.