Which one will be given more importance? A strong recommendation from a new assistant professor or an average recommendation from senior 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.
The effect of multiple recommendation letters isn't additive (usually it's subadditive). So, it's better to have recommendation letters from different types of professors who know you in different ways. A strong recommendation from a new assistant professor could carry a lot of weight, but they don't have a lot of experience to compare your work with other students, which a more experienced professor would. So you do probably want a letter from a senior person who has worked with a lot of students, but luckily, you're allowed to get letters from multiple people, so you don't have to choose.