Recommendation letter from assistant professor vs professor
You will want a recommendation from someone who is familiar with your work and who can speak to your strengths. Overall, I think the content of the letter matters more than the exact person writing it, provided s/he has the qualifications to write the letter in the first place (e.g., a professor or someone otherwise qualified to assess your academic and research work). 

If you have had considerable interaction with the new professor through research, class time, et cetera, then s/he would be a good choice for a letter. Probably the only time you may even consider going with the senior professor (and weaker letter) is if the absence of the letter from him/her would be a red flag. For example, if the senior professor was your advisor, most people would find it concerning not to see a letter from him/her.