50MHz has the advantage of less people on it, but I'd check prices and availability of the 50MHz equipment. 72MHz is the most common, so getting a good deal on new and used 72MHz equipment will be much easier, and I think you'll have much more brands and models of equipment to choose from in the 72MHz band. I haven't looked for 50MHz stuff myself, but I know that 75MHz surface versions of the RXs & TXs are much more expensive, and there are much fewer choices.