The 42 watters produce more heat than the 26 watters by a lot. Although maybe 18 26 watters is comparable, I think it will get pretty hot in there. Be ready with a 'light' to 'medium' powered 4" intake and outake fans, and another fan on low or medium osolating within the chamber. Although all this information depends on how confined the space is. Open space = easier to control heat, confined space = heat can spike during hot whether, and will be generally hotter.
Also a fan blowing over the roots of the plant will help fight mold. in general the more circulation the less problems.
For example high heat will cause mite and other infestations to reprocude more quickly, potentially turning a mild problem into a bigger one. Therefor running a little on the cool side is better, especially if you know you could get heat spikes.
Hope this helps, peace bra.