Excellent reasoning. Does sp_updatestats perform a FULL_SCAN or 50% sampling ?
Good question! sp_updatestats will run an UPDATE STATISTICS statement on all statistics in the database. This will use the default sampling that UPDATE STATISTICS would use if you ran that instead and did not include any additional parameters or options.
The default sampling of UPDATE STATISTICS is based on the data distribution and type, and therefore is not uniform. You can pass a resample parameter into sp_updatestats, but all that does is tell it if it should use the same sample size as last time or not (default is to not reuse the previous sample size). So sp_updatestats does not use FULLSCAN or 50%, nor can you tell it to do so.
For more granular control over the updating of statistics, use UPDATE STATISTICS, where you can specify more options,if desired. If the defaults are cool with you, then sp_updatestats is worth the convenience.