Same thing. The only difference is in one you specify the PK name, in the other you get an automatically generated name. In all other aspects those two statements are identical.
As a matter of preference I always encourage architects/devs to ALWAYS explicitly name anything/everything they create that is namable. This will prevent auto-generated stuff from showing up as a difference if/when you compare databases (say across dev/test/prod systems). It also lets you know intelligently what the object really is by a good naming scheme.
Kevin G. Boles
SQL Server Consultant
SQL MVP 2007-2012
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