which one is better?
you can do resource layout comparisons with different field levels on Kirilloid.
From there we can see that with level 16 resource fields (just an example), 15 cropper with 75% bonus produces 64313 resources per hour, most of which is naturally crop. In comparison, 9 cropper with 100% bonus produces 59064 resources per hour, around 40000 of which is crop. In conclusion, 15 cropper with 75% oases is better.
However, it's always good to keep in mind that with 9c you also get some benefit out of other resource bonuses than crop that you might gain from oases, if you can access those 25%+25% combinations. In this case, those kind of oases would not pull the 9c to the level of 15c in terms of resource production but they would shorten the gap.
Hope this was helpful. Feel free to explore Kirilloid for other resource layout productions with different resource field levels.