I like US-49 and think it is the best option. Four lane divided, not limited access, speed limits up to 65 (not carefully observed) slowing in more congested areas. It is kind of suburban, a bit stop and go sluggish out to Richland, speeding up and clearing up by Florence.
Take I-59 around Hattiesburg. You can go through on US-49, and it is an interesting city to see, but might cost an extra 20 minutes to get through most times of day, 30 minutes or more the wrong time of day.
From Hattiesburg to the coast US-49 is mostly rural (runs through national forest) with a bit of traffic from lumber trucks, which tend to run fast until they get to their turnoff.
There's a cutoff to D'Iberville (SR67?) worth taking if going to the casinos or somewhere further east. If destination is Gulfport, stay on US-49.
P.S. if you decide to take US-98 from Hattiesburg to Mobile, come back and tell us about it. I've not done that one for more than twenty years, and my destinations are now further east so I'm wondering what 98 is like these days.
Tom Test
Itasca Spirit 29B