I never thought the Duskblade as an offensive caster. Usually got him a good number of utility spells that would always be useful regardless of the level, and PrC'd straight to Abjurant Champion at 5th level, then at 10th level continued the Duskblade for the full 15 levels, without losing out on anything precious.
However, you could potentially go: Duskblade 4/Wizard 3/Ultimate Magus 10 for both spontaneous, and prepared casting, whilst being able to cast in Medium armor, and a heavy shield.
Looking at Casting Levels of... Wizard 12/Duskblade 12 before any feats are calculated in to boost those up further.
But, as most of those spells would be used on self- and party-buffs, you wouldn't have to worry about beating the opposition's SR or anything of the like. The only thing that would take a hit is your durations... And BAB, as that's 13 levels of 1/2 BAB.
Personally, I would do the first option.