That's a 5700Watt generator. Either will work. 50Amp will provide you room if you decide to upgrade the generator down the road, however, installation is more expensive. That generator isn't going to run the AC, 50AX240V=12,000W. Do a realistic evaluation of what you MUST have during an outage. e.g. Frig, freezer, etc. When you have that list add up the total wattage and see where you are. Good rule of thumb is small portable units should be run at half load steady state to minimize fuel consumption and maximize life expectancy of the unit, that's 2850W. Also the 2850/5700 numbers are for gas, if you plan on using propane derate by 10-15%
The "Manual transfer Switch" you show will work, but installation may be expensive. Depending on frequency and duration of outages where you are, consider an extension cord. Link attached, there are tons of them out there this is just an example. To me, frequency and duration of outages determines how convenient you will want putting the generator in service.
You're asking the right questions,
Kutatek 40Ft L14-30P to Four 5-20R, 4 Prong Generator Distribution Extension Power Cord, 30 Amp 125/250V 7500 Watts, 10 Gauge SJTW Cable, UL Listed - - Amazon.com==