how much weaker are wizards(and other casters) in this edition compared to 3.5?
Quite a bit. In the same sense that fission devices are much weaker than fusion bombs. You still wouldn't want to be standing anywhere near one when it went off, and you'd still want to refrain from using one in a conventional conflict.
Seriously, though, it's a mixed bag.
3.5 wizards had more spell slots, but had to prep spells in them, so could end up with no suitable to spell to cast in some circumstance and/or with wasted slots at the end of the day - 5e wizards have fewer slots, but use them spontaneously, so can generally pull out the best prepped spell for the circumstance, and they have cantrips to fall back on when facing minor situations and not wanting to waste a spell.
3.5 spell damage scaled with caster level, but saves DCs scaled with slot levels. 5e spell damage scales with slot level, but save DCs scale with character level.
3.5 concentration could be buffed to the stratosphere, making AoOs for casting, trying to cast while grappled, interruption by readied attacks, and the like trivial, 5e concentration is a hard-to-buff save, but it applies to maintainig only a few spells and there's no AoO for casting or interruption or the like, at all.
One area where 3.5 casters are more clearly ahead is item creation, systematic and readily abuseable in 3.5, entirely in the DM's court in 5e.
