There is a lot of talk here about what could be the case and happen, but not enough emphasis on the most likely situations and outcomes. The 725 tubing signifies a higher performance bike. It is plenty durable enough. There is no need to think of it as fragile. OP needs to find out how the tube sets compare in gauge and diameter to know how they will compare in weight and stiffness. Thicker walls make a steel bike heavier and stiffer. Larger diameter does more to increase stiffness than does thickening the tube walls in the usual scenario. So the common compromise is for the diameter to be increased and the wall thickness decreased. That gives very good stiffness at a reasonably light weight. The thinner walls benefit strength wise from stronger tubing like 725 to make up for the thinning. Yes thinner walls and larger diameter tubes are more susceptible to denting than otherwise, but not drastically so. Designers generally stop the thinning and increasing the diameter before they get to that critical situation. If you can afford the 725 bike, that is the one to have. If you can't the 520 bike is a workman-like compromise. Just don't let misplaced concerns turn you off the better product for no good reason.