Iodide is the better leaving group, but the tendency for quaternization (when n-alkylating sec. amines) is higher for iodides. Off course, this also has steric reasons.
HI is easily prepared from KI and phosphoric acid. I just did it two days ago. Use 85% Phosphoric and you can watch the HI formation when the solution turns brown. This way you can easily halogenate your propylalcohol via SN2.
Can u be more specific what substance you want to alkylate?
Speaking of the n-alkylation of 4-piperidone, according to a paper I recently read, they obtained the best results with alkyl chloride, with bromides and iodides giving lesser yields, as well as quaternisation issues. But this might be specific for piperidone chemistry...