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.

A pair of mono attenuators can remove the need for a balance control. Removing the circuitry for the balance control will simplify the construction and inprove the sound by having less complex stuff for the signal to pass through.
Balancing the pair of controls is not hard. (also, some dual mono controls are on a single shaft (concentric shafts with one knob in front of the other.) in back, true dual mono design with each channel separate.

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 The Swiss used to make very good ones but my knowledge of them is out of date.