It really depends on what you want to do with your stint in consulting - few years then another firm, few years then in corporate, stay and go up the ranks etc. - and assess the company culture regarding your plan (even though it will most likely change over time :))
From a reputation perspective S& is slightly better recognized in Paris as a separate strategy consulting activity, whereas Monitor is more blended within EY galaxy of service lines. From a project/industry perspective it is something very dynamic, especially because these 2 firms are agressively hiring partners, so the projects in 6 months can be quite different from those that are ongoing today.
Finally the paycheck should not be a criteria in my opinion, but that's really just me :)