I'd use cap strength (Tsomelevel - Tparcel) AND CINH... The "cap strength" is usually the max temperature difference between the temperature of the environment at a particular level and the temperature of the parcel at that level below the LFC. CINH is kind of like the opposite of CAPE -- it's the area between the temperature trace and the parcel trace where the parcel trace is cooler than the temperature trace. In other words, it's the amount of energy require to lift a parcel through a capping layer.
So, why use both? Let's say you have a tall, skinny CINH profile. In other words, suppose that the temperature is just barely warmer than the parcel trace. In this case, the "cap strength" may be quite low, since the Tlevel-Tparcel is low. However, if this is the case over a deep layer (say, 900-600mb), you may not get initiation unless you have deep convergence. In another case, suppose that you have a sharp cap, where the temp at 800mb is 7C warmer than the parcel at that level, but the cap is shallow (steep inversion). Such a profile may have the same amount of CINH as the previous example, but the "cap strength" would be much higher. Despite this, very strong forcing may be able to lift surface parcels through that capping layer enough to initiate and sustain deep convection.
Here's how I use it -- if there is a sharp inversion, then chances are good that the "cap strength" and CINH are pretty significant. In addition, a little warming /moistening of the parcel layer may not remove the CINH / capping very quickly. On the other hand, if there is no sharp inversion, the "cap strength" may be low, but we don't really know a lot about CINH since we don't know the depth of the capping layer. If it's deep (like the first example in the previous paragraph), you may need deep-layer convergence to initiation convection (e.g. not a shallow outflow boundary). However, a deep, skinny CINH profile is usually more easily removed by warming / moistening the low-levels or cooling the capping layer.
I'm not a fan of using 700mb temp to determine 'cap strength'... The 'cap strength' is a function of both the environment and the parcel (in that the cap strength is the relative difference between the two). 15C 700mb temps may be 'too warm' for convection one day, but 'not warm enough' the next (if, for example, the low-levels warm / moisten). There are some rule-of-thumbs out there, but I've never really followed them.
