I am making a circuit for measuring AC mains voltage and current
1) If your looking to accurately track the AC mains voltage (L-N) over a wide range an isolation transformer will give you poor non-linear results. As the AC voltage drops lower the transformer core transfers energy less efficiently, so your low voltage readings are actually higher than your meter indicates.
2) To get accurate voltage measurements use probes with differential inputs, rated for 500vac or twice the voltage your testing. They have a common ground which can be tied to earth ground if not done by design. I used that type of setup to measure the voltage and current on a 0-30KVDC wire used to charge a capacitor bank for surge testing.
3) For voltages below 1,500vac you can use analog-isolators such as those that Analog Devices make, if you do not mind the $30 USD cost. If your tracking an AC voltage over a narrow range then a transformer is good enough.
How is using a transformer different than using a voltage divider?
4) Avoid voltage dividers because you do not have galvanic isolation from the AC mains, where as the methods I mentioned do, including use of a isolating transformer.
5) Think about what your needs are, and ignore the 'he said/she said' statements, or you would be to scarred to build anything. Be careful no matter what method you use. If you use a transformer use a fuse on the primary in case of things going wrong downstream.