It sounds to me like you don't have a relationship with either Professor. If so, don't quote the first one. Don't even ask.
There are other ways to get what you want. The first Professor gave you an opening. If you follow through on his advice, you have a means to build a relationship with the guy. Mentors actually love it when others take their advice (since it doesn't happen as often as they'd like). So if the advice seems worth pursuing, go for it. Just think of it like a friendly game of tennis. It's only fun when the other player is able to return the ball back to you. So return that ball (if you think it's worth it) and keep the exchanges going.
As to the second Professor, there are other ways you can get feedback from him. And you're right, email is horrible if the guy doesn't know you already.
If he's located in your general geographical area, you should drop by his office. If that Professor can associate a face to the email, it's much more likely he'll take your email much more seriously.
Another thing you can do is frequent the same mailing lists and the same discussion forums this Professor contributes to (assuming you can guess which ones they are). If you become familiar with his research and his computer science-related interests, that's another way to grab his attention. If you provide good feedback on some of his ideas/projects/posts/code, he will most likely do the same for you in return if you ask him.
And by the way, do not fake an interest in his research. Study his research/interests until you're genuinely interested. People can smell fake enthusiasm. So don't fake it if you're not really interested.
And finally, don't be afraid to ask for referrals to other academics if a particular Professor reads what you have to say, but is not interested. The only reason I'm not suggesting you do that for the first Professor is because you don't seem to know him and he does seem interested. He just thinks that your project should be going in a different direction, so he's not likely to refer you even if you ask him (without you first incorporating his initial feedback into your project).