Both units are 5w x2, for a total of 10w audio power. The 450 may project the audio better for someone sitting in front of it, but for the application you're describing, the difference may be minimal. The sound should reach comfortably, and the volume would be somewhat dependent on the hearing aid.
Unfortunately, I'm not sure if this answers your question, but it may not be a question I'm able to address properly.
I was just about to order the 26" C350 Samsung when I noticed I could get the C450 for just a little more. The problem on deciding is that this for an older person with a hearing aid. We decided to try and get one with front speakers ( which the 350 I believe has). The 450 has bottom speakers right? Will the 350 front speakers project just a bit better? Picture quality may be better on the 450 but that is not the concern. The ability for them to hear it clearly without having to raise the volume is the priority. Thanks for any info as I have to order one right away.