I would say that this depends on your ability to make multiple 5* items (financially) as well as how far you are in T2 and also if you ever plan on visiting the Fire Storm Citadel.

For now, due to the patch and your topic, I will assume that you would only like to choose one bomb. If you are still mid T2, the Ash will simply make your runs much, much easier due to its ability to DPS a huge radius. The only situation you wouldn't use the Ash is obviously oilers as stated above; however this encompasses a very small portion of runs. It is a very excellent damage dealer. The Shivermist in T2 is not really as useful. Arenas and danger rooms are where they shine but otherwise you don't really need it.

If you plan on continuing to T3 the Ash is still viable; however if you plan on replacing your Jelly runs with Fire Storm, obviously you cannot bring your Ash. A Shivermist in the Citadel is very, veeeeery useful. Therefore you must look at your endgame plan and your financial ability. What would you like to do at T3? Would you like to grind the Citadel? Can you invest in an Ash now, and then in a Shivermist later, or perhaps in an extra weapon slot?

In conclusion, if you only have the ability to make one bomb now, and would like to endgame in T3 and the Citadel, a Shivermist would be the better investment. If you are a ways to go to T3 or don't plan on the Citadel, then the Ash is the better investment.

In my opinion the Shivermist is a better bomb as its practicality is limitless. The Ash may deal DPS but the mobs will still chase you, and you still have to dodge. The Shivermist will immobilize the mobs so you can literally almost stand in one spot, or at least the dodging will be easier. Your teammates can focus down the unfrozen or deal charged damage to the frozen- a respite from mobs and free swings at their backs is worth more than the DPS from the Ash.