Looked at the Excel 365 model, it's not a aluminum pannel flooring, it's a air mat floor, personally have tested may sizes air floor sibs, although correctly inflated to it's recommended working pressure the air floor still rocks like a thick marshmellow. Don't you think that air floor has same aluminum rigidity, having said this, a 340 will rock a bit less than a 365 because has shorter floor lenght. Have inflated an air deck to 12 PSI and still rocks, that's the counterpart between portability versus floor rigidity. Can't have both at the same time without affecting performance and boat behaviour.
A 340 is a nice lenght sib for 2, can be car topped very well as won't have more width than 1.50-155 cm. A 340 weights a total or around 45 kilos and sib + floor comes in just one big bag, a space saver. Don't transport deflated bagged sibs on car top, can be real dangerous to other drivers spacially on highways, use car trunk space.
To have an idea, have recently moved form a 360 sib to a much larger 380 sib, this sib is used mostly of the time by a 2 mate crew, now experimenting some bow rock issues because the sib needs to have all weight toward middle/bow to compensate, and we are definitely no light mates. Will ned to buy a tiller extension so to move my weight towars bow. So, the longer the sib, will ned more mates to compensate, unless you have at hand a junior sumo fighter siting on bow.. A 380 is to big for 2 specially if they are youngsters or medium aged mates, and the engine used, it's another story... It's your final decision if opting for a 340 or 365 model.