Surely neither is better, they're just different... different set ups suit different people. I know a few people who have stepchildren and children from a previous relationship and all is great, but obviously the more people involved the more complicated things get and the more potential for issues.
Pluses from my totally inexperienced hypothetical only position - having a larger family without having to go through birth as many times 
, if the children get on then that's nice for them to have stepsiblings, especially if you wanted to have children close in age but your relationship ended, for example, or a first child might benefit from gaining an older stepsibling.
If contact weekends match up, then you potentially have a mix of time spent with children and child free time. If ages are appropriate, you might have some free babysitters as long as you didn't abuse this all the time.
It's probably easier to discuss parenting related things with someone who is already a parent and has experience of parenting, and they might be more up for mucking in with your DCs rather than dithering on the edge, although of course this could be a disadvantage if their parenting ideas are very different from yours. However, at least if they've already done some parenting you can discuss these things before moving in, rather than the difference between the ideas we often have before DC come along and then the reality.
Also, it's a good sign if a man has contact with his DCs, or at least, it's a red flag if he doesn't see them.
As I said though, totally hypothetical and ONLY focusing on the positives of new relationship plus stepchildren rather than new relationship or no relationship.