It's better than MS because it stacks Sorc and Wiz levels (or whatever else), not just spellthief and X levels. Though it only applies to one school.
Erm, that's not how Master Spellthief works, here's the text on it:
[spoiler]
Your spellthief levels stack with levels of other arcane spellcaster classes (that is, levels of any class that grants arcane spellcasting other than the spellthief) for the purpose of determining what level of spell you can steal. For example, a 4th-level spellthief/4th-level wizard could steal spells of up to 4th level, as if he were an 8th-level spellthief. Your spellthief and arcane spellcaster levels also stack when determining your caster level for all arcane spells. The character described above would have a caster level of 8th for both his spellthief spells and his wizard spells. In addition, you do not incur a chance of arcane spell failure for arcane spells cast or stolen from other classes, but only if you are wearing light armor. You incur the normal arcane spell failure chance when wearing medium or heavy armor or when using a shield.[/spoiler]
In bold it tells us that your spellthief and all your arcane spellcaster levels stack. Granted, this other feat gets you the ability to stack your cleric stuff as well, but only for one school, and you don't get the added effect of casting in light armour.
You can always do both
What would my caster level then become?
Say I'm a Spellthief1/Sorc 1/Wizard 3/Ultimate Magus 10. Casting spells from my speciality school, I'd reckon it'd become a CL of 47?
What if I stuck in some levels of sublime chord? Or knight of the weave?