Some points on each:
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M109:
more room in the shelter since it is the bed-body and not a separate container that fits inside the existing bed. It's about 8" wider and 8" longer than the s-280 shelter.
has windows
usually has ''hobbyist starter kit" electrical wiring as well - a fule box and 120V/24V hookups.
should have a ventilator exhaust fan
made of steel over wood. wood subject to rotting if the steel cover has rusted or leaks, making rust and leaks worse over time. Floor planks may rot if water pools in floor, causing later rust out of floor.
If the walls (inside, the wood) is in good shape, you can mount things wherever you want.
has wide door on back, in case something really big has to be put in.
Some have lift gate.
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M35 plus s-280-like shelter
shelter held on bed by mounting system of easily accessed cables. removeable with 5-ton wrecher crane.
S-280 type usually made of aluminum square tube skeleton and skin with foam insulation about 2" thick inside.
floor has skids underneath, giving a few inches clearance under the shelter in the M35 bed.
Has no windows, may have an escape hatch in the door. Fairly airtight. Might want to use a demudilifier during humid/winter months.
When buttoned up, fairly RF tight.
may or may not have electrical inside. I was lucky, mine had a full and complete setup.
There are places you can mount things to and places you cannot. They are usually obvious. Mine has plenty of mount points.
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In closing, try to get a look at a specimen of each kind you are considering. I had origninally looked for an M109 (the windows are sort of nice), but the M35+shelter has worked out well for me.