Transaction comparables are usually used to value companies for the purposes of other similar transactions, so they tend to be more a concern of investment bankers rather than portfolio investors. However, essentially the same technique can be used to evaluate the potential for upside from possible takeovers.
The use of transaction comps, differs from using trading comps, in that the value is based on the level at which transactions have occurred, rather that the level at which peers trade. In other ways the methodology is much the same.
The peers available for transaction comps differ from those available for deal comps, so one method may be usable when another is not. Transaction comps use recent deals which may include those in listed companies.
Using some types of transaction (such as IPOs) will lead to a lower valuation than trading comps, while using others (such as takeovers) will lead to a valuation at a premium over trading comps. Which is preferable depends on what the valuation will be used for.
Apart from trading comps, the main alternative to deal comps are DCF valuations.