I have a samsung s3 and i have the prepaid plan now and i got a txt from koodo sayin that i can switch to a monthly plan and possibly get a newer phone to boot!! Which i kinda could use since this one has a few issues
Everything depends on your needs and financial situation. If Koodo offers this to you it may be something worth consideration. Don't rush check what is in the offer and decide after that. S3 is still OK but has some limits and if it gives you some problems it is time to change it.
Buy a new unlocked phone from another vendor. There are lots of sales going on with Black Friday and the holidays. I bought a handset from the US (nothing I wanted was sold in Canada) and purring along on my prepaid (since 2013). I have received these offers for the last couple of years but they have never been worth it to me.