Hi Tom
I have no experience of the Hammerli but I do of the LP400 and of the Walther company in particular. The only review I can find on line of the Pro is by an inexperienced pistol shooter so is near enough irrelevant.
The difference is, the Walther LP400 Club is exactly the same as the top line LP400 match pistol, just with a cheaper grip, the Hammerli Pro (built in a different factory) is a designed and built to a price budget pistol with a top line grip. Look on the Walther web site to see the technical spec of both.
My advice to those buying new kit is always get the best you can afford, buy once, buy the best you can afford and develop into it. The Pro will always be a budget pistol with a top line grip, the Walther will always be a top line match pistol, and remember most shooters when they develop, either modify the grip to them, or have a new grip made for them. The differences noticed by a top level shooter will be shot recoil and vibration feel (even recoilless pistols recoil!), and trigger feel, initially especially coming from an eastern Bloc pistol you will be impressed by both, but as your skill develops the Walther will be ahead.
Have Fun
Robin