Chem PhD -- LoR requests : who / different dept.??

[KaffeeCafe]

Hi all,

I just wanted to get a general opinion about who to ask:  Is it more ideal to stay within one's study/science in general when choosing LOR?  I have a Honor's professor (high up) who has written me a recommendation before and I know likes me quite a bit, so I was either considering him or other Chem profs I had and did well with -- 2 of which have already written in the past and I've had another offer directly to write one if I ever needed anything.  I already have two chemistry ones in mind at my home institution: my research advisor for 2 semesters/prof for 2 semesters (with whom I have a paper published) and probably a professor I had and did possibly the best in the class, and I was also a 'supplemental instructor' in (attended class and held formal review session 3x/week).  I also may ask one of my REU PIs (who is a known 'name' I would say and who has written one for me before), but it's a little awkward because he has followed up with me more than once about going to grad school with him, and it's not really my first pick so maybe he wouldn't like writing for me to other schools.

So, I think I have enough options -- but was curious if they should all be 'science.'  Obviously I want one from my primary research PI and a professor who has class experience with me.  

Thank you -- I'm hoping to ask this week.