Opinions on Simplicity built lawn mowers

We have a Ranch King Lawn mower and I really like the way it mows.  Very nice smooth finish.  Lately the tranny has started to act up and we are looking at replacing it.  A local mower shop has a 4212 Simplicity and a 5216 simplicity (both hydros) on his lot.  I have also found a 1976 710 Allis lawnmower for sale locally as well (although the owner says it is a 16 hp kohler instead of a 10 like simpletractors.com says it should be).   What are everybodys opinions on these mowers?  I always liked the single cylinder briggs engines like the 4212 has, but the 5216 has a 16 hp briggs twin cylinder opposed engine which to date I haven't had good luck with (could have had a bad engine though).  The 710 looks ideal to me, just I haven't got a chance to look at it yet.  What is everybodys opinion on how they mow?  I used my uncles newer (1992 style) Simplicity Landlord and it does a terrible job of mowing in my opinion, but the deck seems wore out and the small deck wheels are wore out to non existant to flat on one side.

Is the 16 HP twin a Vanguard series,(V-twin) or "boxer"-style(180 degree opposed)? I have a 16 HP Briggs twin-2. It has been running since 1986 with only one burned out head gasket replaced since new. No other internal repairs have been made. Very few other repairs to engine. Occasional carb rebuild needed. Had to replace the charging system stator and regulator unit 3 years ago, that's it. Still has decent power, no oil cumsumption problems. Did I mention it still mows over 2 acres at my place, and almost that much at my sons' place, and that's the least it has been used since it was new. Tractor has been a good one too. Snapper YT-16, 42" cut. Replace deck spindles periodically, mower belt every 3 or 4 years. No other major repairs. Did weld a band of 1/4" x 3" plate all the way around the deck. Good engine, good machine. I have a 16 HP V-twin on another machine. The overhead valve engine gives quicker power than the boxer, but it is not nearly as trouble-free. 

I bought a new 1616H AGCO in 1996 and it's been real good to me and can really go through grass compared to Cub Cadet. Doesn't compare to some of the commercial front cuts but it does good. It's mowed a couple acres from 1996 until about 2007 when I got a C with a Woods. I replaced one belt and the PTO switch and that's it. Now I just mainly trim with it.

Make sure you have a fuel shut-off on those 2 cyl engines, that may help out your carb problems.