Let's not forget, however, that Final Fantasy IX lets you change your job as many times as you want, with no penalty (Save that if you level your jobs at different rates, you may need to keep lots of additional weapons and armor in your mog house)

What you could do is level as a monk up until lv 7 (When you get access to your first series of bigger and better weapons/armor), and then switch to THF and level it to 7, etc.

Another thing to keep in mind is that in FFXI, you can have a subjob after Lv 18...So you could have MNK as your main job, and THF as a subjob. Hence, when you're LV 30 as a MNK, assuming you kept up levelling your subjob, you'd also have the abilities of a LV 15 THF, including Treasure Hunter.

Even before getting a subjob, you could always change jobs at your Mog House and only play as a THF when you want to go gil farming.

Personally, I find THF more enjoyable to play, but my advice would have to be to use MNK as your main class and THF as your subjob, because the MNK's 2hr ability is absolutely effing AWESOME. It rules as much as Chainspell (RDM 2hr) sucks.