I had similar thoughts. I'm currently with internode but recently signed up to a further 12 month plan so couldnt break it.
Primus does seem pretty competitive price-wise and I believe the service is good. It's not so cheap that I'd switch at this stage though. I will definitely consider them in the future.
I believe iPrimus have more of a focus on the business customers. I recall reading somewhere following the M2 takeover that M2 were not anticipating any increase in individual non-business primus customers (not sure where I read this).
MTU Price at posting:
$4.19 Sentiment: LT Buy Disclosure: Held