If you're looking at a back-inflate design (good idea, BTW), and looking at spending 300 to 500, I would (and did) go with a Backplate and Wing. I'm not a super experienced diver by any means (80 dives over the past many years), but I really, really, really like my Backplate/Wing setup a lot more than any of the rental jacket BCs I've rented in the past. Add ditchable weight pockets and you're good to go.
The Explorer is big, bulky (no easy packing there) and heavy (4.2kg, about a kg heavier than my BP/W setup with Al plate), has a huge amount of lift (total overkill), and very likely positively boyant (meaning even more weight to drag around with you).
Where are you located? There are quite a lot of vendors in the US that can get you a very nice BP/W rig for a fair price (Jim Lapenta for one, Tobin from Deep Sea Supply for a little more cash, but more options in terms of plate sizes, etc.). Advantages are modularity (easy upgrades and repairs), better bouyancy of the rig (carry less weight), and fewer frills. If you've got good mobility (no shoulder problems), I'm willing to bet you'll find a well adjusted single piece harnass more comfortable as well (I certainly do).