I would just go ahead and put in the NEMA 14-50R. In the grand scheme of things it won't cost you any more than the 6-50 and you are essentially future-proofed for the foreseeable future. You night as well save yourself a little trouble down the road.
My personal preference is hard wirea 48 or 50 amp station. This future proofs you if you get a full electric vehicle that will need quicker charging. The plug-in stations are limited to 38 or 40 amps (I think)
I also would go with a 14-50. It would be possible to use an adapter to connect a 6-50 plug to a 14-50 receptacle but not vice versa. The 14-50 has a ground and neutral (requires 4 wires) while the 6-50 only has a ground (requires 3 wires) so in general the 14-50 is more versitile if a 4-wire system is ever needed or codes change, etc. While a 6-50 can be used with 3 wires the cost and space of the additional wire for the 14-50 is negligible in the bigger scheme of things IMHO.
Whether the unit is plugged in or hard-wired doesn't change the 80% limitation of the circuit. A 50A circuit can only be used to supply 40A for continuous loads and it doesn't matter whether the unit is hard-wired or plugged in. The NEC uses 125% instead of 80% but the result is the same. Sizing Conductors, Part III