Both conditionals are IMHO nonsense, you don't want to compare a formatted int with an int as that would give unexpected results, as for example '1.000' > 10 is false ('1.000' is 1000 formated with . as thousand separator).
So what you really want is
Code:
<xf:if is="$xf.visitor.message_count > 10">
The first version will work everywhere and check if the viewer has more than 10 messages, the second one will only work where $user is defined and check if that user (whoever that is) has more than 10 messages.