Parentheses cause the commands to be run in a subshell.
Braces cause the commands to be grouped together but not in a subshell.
Given that your example does not use side-effects, there is no real difference between both. If there were side-effects, e.g. setting or modifying shell variables, there is a difference as such side-effects applied to a sub-shell will be forgotten when this sub-shell ends.
If you however take a closer look and compare the behavior of different shell implementations, it becomes confusing:
The Bourne Shell e.g. runs grouped commands in a subshell in case there is an I/O redirection and ksh93 avoids subshells by implementing virtual subshell behavior that is done by creating a temporary copy of new parameters. Whether this is always 100% correct is not known,
ksh93 Version M 1993-12-28 s+ from 2009 e.g. implements $(...) incorrectly and $(alias a=b) affects the main shell.
So in general: if you are interested in specific aspects, be careful and check your shell for it's actual behavior.