Electrician and welder here,
the only reason to run a 200 amp service to your garage is if you are planning on welding aluminum, and thick aluminum at that. if you are a hobby welder, think anything below 200 amps(welding amps) only pulls 54 amps out of an inverter based machine, the old tombstones or transformer wound machines will pull more but I don't see you welding with 5/16ths rod.
I ran my entire house, and my garage off of a 200 amp service and I welded professionally for years, if I had anything big or thick I'd run my engine drive and it would push 300 amps all day.
that said, after wiring and building several garages, my ideal setup for any climate would be a 100 amp sub panel with propane 99+ EFF furnace, and maybe a 3 ton ac unit for cooling should you require it.
the 400 amp service be it a 400 can or a 320 can doesn't matter, you will likely never come close to pulling 200 amps regardless unless you have 2 hot water heaters, Tesla charger, and base board heaters along with 15kw heat strips in your air handlers... so without a full audit of what you have I couldn't make a professional judgement, but I do install upgraded services frequently, 1 out of 10 or so, switching from 200 to 400 because they both bought teslas etc...
BIGGEST thing in my opinion is run a copper 2/0 to your garage to reduce voltage drop. one thing people don't realize is that most small houses have around 3000 feet of wire in them, it averages out to 2 feet per square foot usually, and that length has resistance, any load that is always on in your house, is always going through a resistor, and creating heat out of your money, so upgrade your wire size on ANY LOAD that is always running, fridge, ac, lighting circuits that are always on, and you will see a difference in your bill, and it will continue to pay for itself for the entire time you own your home. this is especially true for hot water heaters, etc...
that said a "SENSE" device from amazon will help you determine your actual KW load and KWH through the day, I have one, love it, simple to install, easy to use,,,... look it up, little red box
AND finally, SQUARE D equipment is the best for homeowners hands down, homeline or QO,doens't matter,homeline has a nicer neutral bar setup, but qo is industrial grade sort of. "caveat there,,, won't get technical here"
PM with questions if you really want to chat otherwise Cheers!!!
