They are both good guns.
They both have a proven record of reliability, and service life.
However, in terms of workmanship, and design, from an engineering point of view, the Mauser 98 is altogether better. The Mauser action is one of the simplest, most reliable designs ever.
The Mauser at one point spanned the globe in use, before and after the world war era. During the Boer War, the British were facing Mauser rifles in the hands of the the Boers. The British were stunned by the accuracy and usefulness of the Mauser. The Mauser rifle nearly ended the Lee Enfield rifle prior to World War I, as the British started to develop a Mauser pattern rifle. The design was not ready for the First World War, so the Enfield became part of British history, not a Mauser.
Another example is the Model 1903 Springfield. The rifle is a very good copy of the Mauser action. So good a copy, that the United States payed patent infringment/royalties to Germany prior to World War I.
Finally, the Winchester Model 70 action is based, albeit loosely, off the Mauser action.
Would I hesitate to use a Mosin, no...but I would choose a Mauser every time given the choice.
VIGILIA PRETIUM LIBERTATIS
"The price of liberty is vigilance"
America is at an awkward stage. It's too late to work within the system, but too early to shoot the bastards.