Other advantages to Propeller in addition to what you say:

  - DIP form factor, easy to breadboard

  - Very little supporting circuity needed. Very easy to set up.

  - Enthusiastic intimate helpful community on the forum

  - GCC available (not an advantage over Arduino but it exists you don't have to use SPIN)

  - Pretty high performance compared to Arduino -- 80mhz on 8 cores [tho that doesn't tell the whole story since memory accesses to the shared memory take a lot of cycles)

  - Small memory space.  32k of slow shared memory space, only 2k per cog (core)

  - New Propeller 2 is taking years to come out and the old Prop 1 is really starting to show its age