I have been a follower of Nurgle for 4 years now, with the new codex I must say I'm impressed at how the fallen legions, DG, WE, TS, EC have changed, making a decision based on these is not going to be easy. I will still play as Death Guard and find them to be the most resiliant basic trooper in the game.
It really depends on the style of play you want, rock hard resiliant marines that can get back up after severe damage or high initiative troops with some of the best assualt weapon upgrades available to them.
My suggestion would be to organise a few proxy games with someone you know, so that you can get the feel for both armies. Tactically and stat wise they are very different and only you can really make this decision.
But if I had to give you advice, go for Death Guard!
Dark Pariah