There are pros and cons to both. In my own experience and opinion, I will only be buying the pills after I am done with my stock of liquid Tamox.
Liquid
Pros: Cheaper, no prescription needed
Cons: Taste like crap (your research animals don't like it so much), dosing & content is questionable, less convenient
Pills
Pros: Dosing is accurate, more convenient, no bad taste (to your rats)
Cons: More expensive, need prescription (unless ordering from overseas)
I don't know what the hell I'm putting in my rat's body with the liquid stuff. I'd rather pay a little bit more and be sure I'm getting the right stuff at the right dosages.