Two Mono Attenuators VS One Stereo Attenuator

The best is stepped - a precisely resistor network - but without knowing your budget or goals it is hard to say.
The best may depend on your applications...do you want to contorl everythig with softwatre for example?

2 mono attenuators will make it difficult to level match - better to get a precision device.

I have two custom preamps, one with Stereo and the other dual mono with each attenuator on its own side. Theoretically the mono is better , turning the two controls is sometimes a pain. Even though the dual mono has better output jacks( I supplied WBT) and is a simpler circuit, leaving out tape loop and having only phono and one line input, I actually like the sound of the other better[ these were built by same person]. So you never know. Stepped is better if the steps are not too course but there are some good smooth ones as well. I have an Audio Synthesis Passive [PAS-02 I think] that has a very good one whose name escapes me just now. The Swiss used to make very good ones but my knowledge of them is out of date.