I’m probably quite late for this party but as I also upgraded from a Spartan Trainer to the Suunto 5, let me just chime in for whoever has this same question in the future.
downside:
vibration is weaker
it doesn’t sync to Movescount (but by now the Spartan Trainer probably doesn’t sync to Movescount anymore either)
positives
you can now actually use the vibration as a wake-up alarm (both in absolute time as well as relative to sunrise in case you want to wake up in time for a sunrise pedalboard session or anything) and even though the vibration is weaker than the Trainer, it does wake me up
you get this visual representation of the sunrise and sunset times on your homescreen, which I really appreciate
it also shows moon phases (for whoever appreciates that)
depending on which Spartan Trainer you have now, you may not currently have a stainless steel bezel which the 5 does have (painted or plain)
battery lasts a bit longer
there are a couple of SuuntoPlus features which you might appreciate. I personally like the “loop” and “sprint” functions thereof.
Compared to other Suunto watches, it indeed does not have a built in compass (though if you’re moving, it can show your heading which is useful too). The external GPS antenna doesn’t bother me. Like the Trainer, the unit is smaller than the Ambit 2S and Ambit 3 Peak (which I also have). If the 9 offers you something that you really want/need then of course get that. But if you were happy with the Trainer then you’ll likely also appreciate the 5.