Late-round draftees get a bigger signing bonus, and the contract is bigger then udfas, but it's not guaranteed.
In other words, if a guy gets drafted and cut during camp, all he makes his signing bonus. Meanwhile, if a guy signs as an undrafted free agent and makes the team, he gets at least the league minimum (for either the 53-man roster or the scout team). In either case, the amount made from being on one of the two rosters is more than a late-round guy would get as a signing bonus.