If you're worried about there being a lien on the bike, then only title will satisfy that condition.

If you're worried about it being stolen, if the seller has the MSO then it probably isn't stolen (unless they stole it from their significant other). More common than you'd think.

If the bike was recently purchased in texas then it has to have a title. Out of state buyers or transplants often skip the step of titling to save on the sales tax.

A bill of sale doesn't mean anything to the bank, the guys wife, or the police if there is a lean on it or its stolen. If you want to trade it in to a dealer later they won't take it on a bill of sale either.

If you later want to title the bike you have to have the original purchaser do so with the bill of sale from the dealer. It will be taxed on the amount of the sale. So you'll want to get the original paperwork of the sale.

So either buy from someone you know alot about or just wait for one with a title to come along. If you do get one with a title make sure you register it in your name. It only costs $33 to transfer the title between private parties.