TaxCaster vs TurboTax

[pingpilot]

I just found TaxCaster when looking for a way to forecast my taxes for 2018.

I used TurboTax for 2017 and it calculated a tax payment requirement for 2017 of $414.

Using TaxCaster and plugging in the same numbers I used in TurboTax for 2017, TaxCaster calculates a $28 refund. Why the difference?

Also, it calculates a $458 refund for 2018. Using 2018 IRS Form 1040ES, it calculates I need to make quarterly payments of $6600.

Can someone explain the large differences in calculations? 

[tuck]

TaxCaster 2018 shows $600 less owed than my spreadsheet calc. I need more detail to determine which is right