It really comes down to preferences and service. I was with Virgin and had the Optimus V for about a month, Virgin's service kind of sucked here, I get much better service with T-Mo even though I heard nothing but bad about them around here, I've so far, had a very good experience with T-Mo.
I've also heard of a lot of people having a lot of problems with the Triumph, when I had my OV, there were a lot of people saying they went back to the OV even though the OV was basically a downgrade from the Triumph. The Triumph is also $100 more, and unless you grandfathered the $25 plan, Virgin is $5 more a month.
On Virgin mobile you do get 300 minutes a month as apposed to 100 minutes a month, but only 3g data speeds instead of 4g, and right there is where I was definitely going with the T-Mo plan, the 4g is SO much nicer than Virgin's 3g. There are ways to get unlimited calling through your data, and this is going to work MUCH better over the 4g instead of the 3g, BUT I had very bad luck with it, but others have had great luck. Looks like it really comes down to about where you're located. (I know there are other plans, I was just going off the cheap plans most people go with.)
I definitely would not trade my Exhibit II for a Triumph, but in the end it's really all on what YOU like. They are very different in many different ways, the pros on one to me or someone else may be cons to another person and vice versa.
What I would do is make a list of what is important to YOU, then compare side by side which meets YOUR standards for what you want.
I hope I helped at all, all that probably wasn't very helpful.]
Edit: I did select the Exhibit II in the poll, because that is what I would personally choose.
Also, here is a link to compare both phones side by side.
Samsung Exhibit II 4G Vs Motorola Triumph