If you want an SUV, the Suburban is absolutely the best choice.
That said... What do you mean by "struggle"? The 5.4L isn't exactly underpowered, however like all small blocks it has to rev to make power. But the 6.0 does also, and even makes peak torque at a higher RPM.
Basically, if you expect to be able to tow without down-shifting, the 6.0 is not going to solve your issue. In fact, even the V10 is a high-revving motor, and will downshift frequently unless you re-program it (it will pull nicely a low RPMs if you add a tuner).
Another point to keep in mind... A 'burb 2500 will give you about 200-300 pounds more payload over your E150, assuming it is a factory wagon and not a conversion van. An E350 van (or Express 3500) will give you nearly double the payload of a suburban 2500.