My experience is that high caliber arty is only good on fixed targets you need to bombard. Otherwise it takes way to long to call it in (more than 7 min. waiting). The only use I have for 150mm is designate an area in the beginning of the battle and let it rain over there .
For example; if you have to take a town, bombard the approach to town, if you are close to the approach to town, bombard the town itself. If there are tanks in the area, good chance you take them out as well. Also nice to put on trenches since these are very hard to take out.
The 82mm mortar spotter has his shells raining in 1-2 min wich can easy destroy that nasty AT gun, stop an advancing / moving platoon etc. In short, great against unexpected ''flashpoints'' in the open ground. These shells are not that good against trenches and / or buildings (dont use them on big heavy buildings).
The 122mm spotter is the max I love. It does good area damage and it is still acceptable in terms of ''waiting time'', the bonus is that if hit a tank right, it can kill the tank. You need a bit of luck though.
Concentrated defences are the ideal target for this kind of arty. Position your troops during ''arty waitng time'' and make sure you are ready to go once the 1st round of bombarding is over. (you might need to wait another round) Dropping some smoke in there helps you advance
All are great against armored and unarmored vehicles. Only when they they are stationary though.
So I use the 82mm for flexible support and the 122 for area bombardment. Dont forget that you can target an area and adjust fire before or during the bombardment. Wich saves precious waiting time. Ofcourse you cant adjust over big distances.
And really, arty on defenders is great, it can give good help in prying open a solid defence. The only thing it cant handle is a concrete bunker. Although I would not use my 82mm on a wooden either .
So in your case I would take 1 or 2 82mm spotters or 1 122mm.
Hope this helps.