Hi Mr. Racingt, I'll chip in with my personal experiences and opinions, but dont take them as gospel, I'm a crotchety old geezer a bit set in my ways.
First thing to remember, these are 50 plius year old machines. Of the two, the 450 was built to a high quality standard, the 350 less so. Having said that, the 350 K1-3 were a very well designed package, the 450 overly complex for not much more performance. After 50 years and who knows how many previous owners, you need intrinsic reliability, or access to spare parts! The 350 is well served in this respect.
Even so, if you want to resurect one of the imports, budget at least 6-8 hundred pounds just for the usual suspects, silencers, most chromework, tyres, chains etc. I know you guys in the states can pick these up for peanunts, not so over here in the UK. But, of the two, I must admit that the 450 does have more 'presance', but I chose a known good package (having owned both back in the day). I found the 450 vibratory, ill handling and heavy. (It also dropped a valve causing engine carnage). The cb250 was a delight and very perky, easy to chuck around and very reliable. I knew the insides of the 450 pretty intimately, never touched the 250. The 350 import I have just finished gives all the signs of being just as much fun as I remember.