I will approach this from the other angle. Lets make some assumptions first.
- Your out of contract completely, not just on one line, but on the entire account.
- Your new unlocked phone and a subsidized phone cost the exact same thing in the long run.
- You can get the phone you want, unlocked or subsidized.
Now with those assumptions in place you have a few things you need to look at. Most of them are guessing and risk.
First, no lock in is better then some lock in. If all other things are equal, then not be locked in will give you the most flexibility and bargaining power should you want or need it.
Second, contracts are not always a bad thing, often they can work to your advantage. By being "in contract" you have a fixed price. Without the contract the price may change. Maybe as frequently as every month. Contracts also, at times, come with better pricing. So make sure to look at the cost of plans both with and without contracts.
Third, When out of contract you can haggle. "I will move to carrier two if you don't address you high cost of data," for example. Often times this can result in the carrier giving you a credit or better pricing. If your on contract there is no reason to give you a better price.
Finally, credit scores and consumer reports. This will depend a lot on the carrier. Essentially it looks better to off contract with a good payment history, then on a new contract with a good payment history. However this effect is lessened on a old contract with good payment history.
In my opinion, your much better off with an unlocked phone, then a contract. There are upsides to a contract that should not be ignored, but in the end, the flexibility of being off contract in conjunction with barging power usually work out better.