Well either way really, the character is taking a hit just by multiclassing. IMO, the only multiclassing that is truly equivelent is when you don't involve spellcasting. For instance a Ftr10/Rog10 is about as powerful as either a Ftr20 or a Rog20. The same cannot be said for a Ftr10/Wiz10 or a Brd10/Rog10. Basically, when you dilute spellcasting while you may not become less powerful at the moment you multiclass, you will be later on.
The regular Arcane Trickster requires 4 levels of Rogue, not 3, so the "cost" of getting some cool abilities is to deny yourself 9th level spells (unless you plan on going into epic levels, but even then your epic spellcasting will be delayed). There are other hidden "costs" as well, usually a wizard will have a good dex and a good int, a sorceror will have a good dex and a good cha, but rogues need dex, int, and cha to excel.
As for the rogue who decides to take Arcane Trickster, he must first have 5 levels of wizard, 6 levels of sorceror, or 7 levels of bard. By that time, he may have more levels of spellcasting than he does of rogue!
The spellcaster who gains all of those benefits also loses 3 levels of spellcasting, 3 levels of spells known or written, 3 points on his caster level, and 3 points on his spell penetration. In 3.5 it will appear to be even worse because some effects are tied to caster level. For instance to dispel a certain magic you must cast dispel magic at 10th level lets say, well by multiclassing for 4 levels, you won't be able to do that for a while.
Technik