My GW2 account
The paid account has less restrictions, more character slots, and can earn 2 Gold per day for Dailies, plus APs and rewards. I suppose it depends on how soon you are going to upgrade to Heart of Thorns. Getting to L16-18 takes little time, really.
Your choice, of course.
If there isn’t much on your current paid account, like account bound materials of achievement points, Id play on your free account and apply HoT to that. You’ll then have two lots of log in rewards coming your way.
You will also get the level 80 boost when you upgrade to HoT. So if you can’t wait to get stuck into the newer content you won’t have to.
You can still play, for now, on your paid account and reap the rewards, then apply the HoT serial code to your Play4Free account, if you wish to reap double the log-in rewards at that time.
Again, your choice.
If you have any characters on your paid account they will have, or will be getting, birthday gifts. They vary on how old your character is but they are a nice perk to have. You can wiki them to determine if that should be a deciding factor for you or not, but I would take that into account. Getting to 16iah takes almost no time at all, but you can’t make your account / characters age faster for the gifts.