I feel that either solution will work for you. It really depends on the price points and if the value of the 5410's is worth the performance increase. I feel it's a toss up, and that in the end, either solution will be a suitable option.
Solutions Engineer WPEngine - WordPress Hosting, Perfected.
the dual e5410 will be significantly faster than the single x3460. We have a large number of both in production. It's not *amazingly* faster, but the E5410 can do maybe 25-50% more pageviews / second on a heavily cpu-bound website that we run.