Well... I actually like the first version better...but, the issue isn't with merely replacing a sample (french horn) with another sample (harp). You need to work on the articulations in the piece, the attack of each note and each instrument. Orchestral pieces are very hard to make for this reason, wind instruments are very hard to emulate and make sound natural using samples.
Short of recording an actual orchestra, there are some things you can do to get more out of your samples. Specifically, as I mentioned, you'll need to work on the articulation (attacks) of each note, how a note decays (fades out), etc. For example, the flute that starts at 2:03 just doesn't sound natural. Every note has the same attack and it sounds abrupt, and there is no change in volume. I play trumpet, and that is just not how any wind instrument sounds. There are several very intricate nuances to wind instruments' sound that need to be adjusted to make a sample sound natural. Now that is no insult to you, it's just a matter of that this is a very difficult genre to tackle. I'm not sure what software you're using, but you'll have to do more than drop a MIDI track into a synth and change the samples. Depending on what you have, there are some decent VSTs that include wind instruments (WIVI, Mr. Sax, some Native Instruments soundpacks) that you might be able to find demos of. With these you can record the piece using a keyboard and control the type of articulation, level of decay, reverb, etc. It might help to try to get a more natural sound.
I like the idea though. Snake Man as a orchestral piece could be very epic. It'll just need a lot of work to make it sound natural. Good luck!