What slaking

Yes, 15/15/09 of course, but slaking is only there for the lulz and it's not a very good pokemon. Jokes maybe but reality no.

All this extra attacking power will hardly come to use. If you lay him in a gym, you must be lucky that his charge move comes before he is dead (if it is not Play Rough). If it hits, it already has so high attack that it will cause huge damage. Therefore to maximize the number of times a charge move is produced, stamina could help a little.

If partly cloudy weather has any chance of occuring near you, wait until you catch a higher level instead.
If not, yes I would evolve the L28 one. It needs a fair bit of powering to be of any use in gym defense though, if that's what you want to use it for.