So now that you have a full range of opinions... 
There is a sweet spot for panels and service equipment @ 200A. However, in most areas of the US, it is legal to have multiple 200A service panels served from the base of a single service meter, as long as they are grouped together and there are no more than 6 switches to disconnect the entire service. As a result, I'd throw out the 300A option and either go with 200A or 400A depending upon the load calcs.
The other cost to consider is the installation cost by your PoCo. In my case, I'm an owner-member of the electric co-op who provides all equipment and labor to do installs and upgrades; they include the meter pan but not installation of it. As a result, I was unconstrained with cost there (although I think they would have had concern if I asked for larger service without justification). In other areas, there is a fee schedule attached to the service size that you are getting.
Your continuous load, as mentioned, is likely to be far lower than 200A, but that's today. I have a 25 kW generator, and when it was installed I did a load test of every large-scale load attached to my generator panel without it overwhelming the generator. That included 2 electric convection ovens, 12 kW emergency heat strips, hot water heat circulation pumps, both A/C compressors, well pump, full lighting, server & networking gear, my office, and other assorted loads. That worked the generator hard, but once everything was running it came in at about 55-60A (I use load shedding to back the compressors or heat strips off if the power quality diminishes). I then have a second panel to which my non-essential loads are connected -- car charging, hot tub, poolhouse/pool pump, shed, laundry, etc.
With Model S and Model X in the same home, I will have times when I am drawing 120A for car charging (80A HPWC + 40A 14-50) in addition to the rest of my loads. The load calculations easily called for a 400A service when you considered these.
In addition, most PoCo's will do a load calculation and recommendation for you as well. Talk to their engineering department, so they can plan transformer size and such for you, too.
There is a sweet spot for panels and service equipment @ 200A. However, in most areas of the US, it is legal to have multiple 200A service panels served from the base of a single service meter, as long as they are grouped together and there are no more than 6 switches to disconnect the entire service. As a result, I'd throw out the 300A option and either go with 200A or 400A depending upon the load calcs.
The other cost to consider is the installation cost by your PoCo. In my case, I'm an owner-member of the electric co-op who provides all equipment and labor to do installs and upgrades; they include the meter pan but not installation of it. As a result, I was unconstrained with cost there (although I think they would have had concern if I asked for larger service without justification). In other areas, there is a fee schedule attached to the service size that you are getting.
Your continuous load, as mentioned, is likely to be far lower than 200A, but that's today. I have a 25 kW generator, and when it was installed I did a load test of every large-scale load attached to my generator panel without it overwhelming the generator. That included 2 electric convection ovens, 12 kW emergency heat strips, hot water heat circulation pumps, both A/C compressors, well pump, full lighting, server & networking gear, my office, and other assorted loads. That worked the generator hard, but once everything was running it came in at about 55-60A (I use load shedding to back the compressors or heat strips off if the power quality diminishes). I then have a second panel to which my non-essential loads are connected -- car charging, hot tub, poolhouse/pool pump, shed, laundry, etc.
With Model S and Model X in the same home, I will have times when I am drawing 120A for car charging (80A HPWC + 40A 14-50) in addition to the rest of my loads. The load calculations easily called for a 400A service when you considered these.
In addition, most PoCo's will do a load calculation and recommendation for you as well. Talk to their engineering department, so they can plan transformer size and such for you, too.
