Looking at pictures from the reviews & the measurements, the Big Boss is slightly longer & girthier. The Tiger has the textured "stripes" and some folks don't like texture, so the Tiger may not work for them. I just got the Tiger a few days ago & like it a lot. The Boss is on my wish list, just for variety on those days I feel like something more...uh...filling.
From the reviews I've seen from folks who have multiple Fun Factory toys, it appears both have the same motor, so the power & functions should be the same. If you were only able to buy one, I'd suggest picking the one you think you'd enjoy best in terms of the size or texture. If you already enjoy textured toys you'd probably like the Tiger. If you prefer size, you'd probably like the Boss better.