Hard choice for me I love them both. Both are a very good standard but I suppose Salt Lick has the edge on design with its mushroom style houses. Kilaguni has the bonus of a swimming pool (cold I will add!) and views of Kili. Kilaguni is in Tsavo West so you can game drive taking in Mzima Springs, Poachers Lookout and the rhino sanctuary whereas Salt Lick is inside a private sanctuary where you could do a night game drive. There is a swimming pool at Salt Licks sister hotel Taita Hills just outside the sanctuary. We had our best spot of lions at SL - a whole pride surrounded our safari bus rubbing themselves on the bull bars, fantastic stuff.
After all that, if I couldn't swing it to do both, I'd choose Kilaguni and do SL next time.
Both are strategically located.but you can combine both a night in each to be able experience all at ago.