I haven't taken both Avonex and Betaseron. They are both interferons, though, and the side effects are similar. The issues as I see it with Betaseron are these: you experience the side effects more often (every other day, as you inject Betaseron every other day!)
and there is some limited evidence that, because you do inject B every other day, you maintain a more consistent level in your blood stream, and so it is somewhat more effective.
As for having more than one autoimmune disease -- unfortunately (and I haven't been able to find any statistics to back this up) it seems like when you have one autoimmune disease, others come along. Not always (like, I've had MS for 25 years and it's the only disease I have..), but sometimes. And sometimes some issues that were thought to be an autoimmune disease -- turns out that it was really MS from the beginning. But I don't know anything about ulcerative colitis, so can't even hazard a guess if that is true in your instance.
Optic neuritis is often a first early sign of MS. People will get that, recover, and have no further symptoms....until, somewhere down the road, other stuff occurs. Just like you suggested. So yes, it is well within the realm of possibility that you had the ON (I'm assuming it was diagnosed by a neuro or opthamologist), and now the pins and needles, tingling, etc., come along.
...I am not a doctor, nor health professional, and don't pretend to be one, here.....