I am new to this site but not new to the addiction of belt feeds. I own both of the guns in question. These are both fully assembled running guns? Got a picture or part number of the 1919 mount as put a better value on it? On the surface $3500 looks like a good deal for the 1919.
If you have a choice between the two I'd definitely go with the MG34 as that is the more rare and valuable. Parts kits alone for the MG34 are going around the $2500 and up range and that is without a receiver. TNWs current price for a semi receiver is $1950. MG34s are less and less available these days. The 1919s are still readily available.
Things to consider- MG34 is ammo. 8mm is not near as abundant as the 7.62N. There are conversion kits for the 34 to shoot 7.62N but more money. The 1919 is a less portable. 1919 is an American Icon. 1919 parts are cheaper, New barrels are only $94 @ What a Country.
What Milford are you from, the one near Scranton?