I agree that Beguiler has a fine spell list and nice synergy with UM (or wizard, or any other INT-based spellcasting class), because INT-based casting reduces MAD. But many of the class benefits do not come into play: the beguiler is balanced against other classes with his spellcasting, plus 1d6 hit die, a fine skill list, the ability to wear armor, and several other level-dependent abilities (metamagic feats like silent, still spell, cloaked casting, etc.) -- as only your spellcasting ability rises in level once you join UM, the only beguiler abilities a typical UM (i.e. beguiler 1, wizard 4, UM X) would get are armored mage and trapfinding. I don't like the armored mage flavor-wise, especially for an UM, but most importantly, it applies only to beguiler spells, not to spells you cast from other classes. Not good.
On the other hand, if you're going UM, you'd probably want to be a specialist, maybe a focused specialist. In that case, the wider spell list a sorcerer offers can help you to tailor the character's spell list and offset the restrictions you are facing in your wizard casting. If you have dropped evocation as your first school, as many suggest, the beguiler's spell list will not be a big help. That also implies the use of spell trigger items -- as a sorcerer-based UM, you can easily grab that wand of maximized magic missiles from the treasure and shoot. Myself, I'd go for the sorcerer. Unless, of course, you mainly use your spontaneous caster spell slots for metamagic.