Pool/Spa Heater Purchase Recommendations

I'm looking for recommendations to replace my pool/spa heater for my spa. Is a Hayward heater as good and reliable as a Raypak? My spa overflows to the pool and valves at the pump operate the pool and spa independently. The previous heater (natural gas) was a 250K btu. My choices are still pretty wide, from a $500 electric heater to a 400kbtu natural gas LoNOX ($3000). I've read where natural gas is the cheapest operation costs for my area, but the capital outlay is substantially more. I plan to heat the spa only (not the pool) a couple of times per week during the winter. I'm leaning toward a 400kbtu Hayward for the price, but one of the pool salesman I talked to didn't like Hayward. Raypak/Rheem appears to be a quality heater, but more expensive. Is a Hayward as good and as reliable as a Raypak? I like the idea of not heating anything until I'm ready to get in the spa, and a 400kbtu heater should heat my spa in 15 minutes. Thanks

I'm not sure if any of the RayPak heaters have salt-resistant heat exchangers; Hayward has them as standard on this line
http://www.hayward-pool.com/prd/In-G...1_14003__I.htm
and they seem to be cheaper over all as well.

I've got two of those heaters in service on a 200,000 gallon main pool, and a 6,000 gallon kiddie pool. Both units are active from mid May till mid June, and from Sept 1 to mid-September. The kiddie pool unit is now 6 years old; the main pool heater is 3 years old. Both have operated without a hitch.

Before that, my only experience was with Laars heaters that were rarely used, except for one unit on an 50,000 gallon indoor pool.