Small portable generators are an excellent backup generator for smaller systems. The newer inverter type are much quieter and more efficient than the older type generators. Be sure to get one that is large enough and buy a quality unit. Typical running hours per year is under 250 with a correctly designed system.
Diesel generators are robust and long lasting. We have seen many that have been well maintained exceed 30,000 hours. The negative side is the cost compared to propane units. They work well for larger homes and lodges, and are quite efficient. When considering purchasing, stay with a name brand and be sure to get a prime power unit, as there are a lot of lower grade diesel generators for sale at low prices. Typical running hours per year can be more than 1500 hours plus.