I have a Walther LG300 Dominator and have just bought a second hand LG400 FT in the original wood stock.
"FT" is important as it is 16J, although there are 7.5J versions so make sure you check. The 16J regulator is longer with 6 rings around it.
Both are top class target rifles and extremely accurate with superb triggers. I use them for Hunter Field Target.
They come as standard with cylinders that you have to unscrew off to fill. Some people get changed to a quick fill but I have never had an issue with them and the zero has never been affected as some claim will happen.
I love my Dommie, this is my second. It's supposed to be my back up rifle for my Rapid Air Weapons TM1000 although I got my best HFT score of 58/60 with the Dommie in its original stock. It's currently in an old alutec stock which I intend to adapt to fit the LG400FT.
The biggest problem with the Dominator is that its an obselete model. Mine is about 12 years old and spares are getting increasingly rare so, reluctantly, it will be going soon.
I am still getting used to the LG400 FT having only had it a couple of weeks, but have no reason to doubt it will be as good as the Dommie.
If you get an LG400 FT and wonder how important the big airstripper is? I can assure you its "very..."! I tried with and without and discovered the groups definitely opened up without it, especially at the longer ranges.
The biggest issue with the LG400 FT at the moment is that the prices for a new one have gone nuts! The wooden stock version used to be about £500 cheaper than the Alutec, now its around £300 more. I know the new wooden stock is made of a nicer wood than it was originally but even so!
I paid a little over £1600 for mine. (Iactually paid £2000 but it had a bunch of Field Target accessories with it which were no good to me and I sold them recovering £400 back).
I have the original wooden stock which is far from the prettiest stock I have ever seen but its design and handling is excellent.
My advice would be try and drop on a nice second hand LG400 FT and save a lot of money.
Airgunaccuracy.com is THE site for all things Walther LG300 and 400.
Hope that's of help.