Well, SoulSilver is Gen 4, and White / Black 2 is Gen 5.
Out of all generations Gen 5 actually had the most Pokemon, so you'll have to collect the least Pokemon in SoulSilver (at least, in those regions anyway.) It will probably be easier to do Gen 4.
So, if you wanna do the harder parts first, go with Gen 5. If you wanna do the easy way first, Gen 4. I had nearly completed Pokemon White with the help of the GTS, so that might be a good idea.
Hope I Helped. :)
PS. Weird, I've got those exact games as well. :3 xD