Fun to race.... What kind of racing are you looking for and how far are you willing to drive?

if you want buoys racing... the Hobie 16 is your only choice..
If you are short distance racing.... pick your favorite. If all of your racing will be handicap... (Local races...pick your cup of tea)

Boomless is a wife and family friendly feature... Sailing wise...the boat has been engineered to work boomless.... eg the mast will rotate to 90 degrees around for down wind sailing with sheet tension. ( I sailed a boomless rig Dart for years and loved it)

If you are single handing a lot.. the Nacra 500 would be a better choice because you can leave the jib on the beach and the boat will be a bit more balanced.... can't do that with a standard 16 set up. In light winds... either boat will work.

All boats are for sale the day you pick it up... so... I would start with the boat in the best mechanical shape with the cleanest sails. Move on when you want to.

You will be very hard pressed to find another Nacra 500 on the water.... You will find lots of new friends on Hobie 16's... It is tough to learn how to go fast without other boats around you... The 16 class is very active and you can find true experts for how to advice... There are no Nacra 500 experts... its a recreational boat.