If I had to choose, I'd just get an IBM 1391401. The typical "Model M" is that model number. The blue labels and whatever else don't really affect how I hunt for Ms because they're just an indication of when the boards were made. I think the older boards are supposedly nicer but in practice, I feel like bolt modding the boards change the feel more than anything else.
The rivets are important because if they're intact, the board will feel "proper". However, most boards have broken rivets, changing how the board feels. If you bolt mod it, you can tune the board to feel the way you'd like it. Dorkvader bolt modded my SSK and it's the best buckling spring over membrane switch I've tried. I'm very happy with it.
The Model F boards are buckling spring over capacitive and feel slightly different. If you can get an AT F and you're happy with that layout, I'd recommend that board.
The Unicomps are ok but I think the cases aren't as nice. And the keycaps aren't as good. They feel slightly different from the 1391401.
Last thing to keep in mind is that if you have 5 buckling spring boards together, they might all feel slightly different due to condition, cleanliness, condition of the rivets, and bolt mod tweaks.