In the nuclear industry, U-233 is well known as an excellent weapons material [1]. It's simply a matter of its nuclear properties, which are well known.

"The fast critical mass of U-233 is almost identical to that for Pu-239 and the spontaneous fission rate is much lower, reducing to negligible levels the problem of a spontaneous fission neutron prematurely initiating the chain reaction -- even in a “gun-type” design such as used for the U-235 Hiroshima bomb (see Table 1)."

[1] U-232 and the Proliferation-Resistance of U-233 in Spent Fuel - http://scienceandglobalsecurity.org/archive/sgs09kang.pdf

U-235 is the only naturally occurring fissile isotope of uranium. In raw uranium out of the ground, it makes up about 0.7%, the rest being U-238. U-233 is also fissile, but only really made in thorium breeder reactors.

They're both good options, U-235 is "simpler" to make though.