How should an undergrad pick a journal to submit to?
Assuming your professor is not new at advising, he would have the best opinion on the matter since it is surely something he has dealt with before. If you are really unsure about this, express your concern and ask him why he would suggest one over the other, and then make your decision from there. 

Additionally, can you get input from the PhD student you are working with?

If all else fails, you may be able to talk to another professor in the same field to get a second opinion on this.