iiNet is still an independent operation* so don't expect to get the same level of service if you jump to TPG. If iiNet is working fine for you (no issues with peak time congestion etc) I would suggest staying put, because you don't know what you're going to get if you jump ship.

Also weigh up TPG's "unlimited" vs iiNet's quota free content. I get by on a 150GB Internode plan, because things like Netflix and Steam are unmetered.

See if you can find people in your area on WP. They might have advice on which ISPs are better on your exchange.

*mostly
*for now