Some directions to help you to choose your trowel :

The shape of the trowel :
The "Mark I" are the one most often used by archaeologists. These trowels are very reliable on the excavation. The lozenge shape offers an ideal edge for the cautious digging of hard soils or stony areas. The improved archaeologist's trowel are the W.H.S. trowels.They are lozenge shaped like the "Mark I".  Imported from UK,  they are considered as the "Rolls Royce" of the european trowels. 

The association between the lozenge shape and a 1,5 mm thickness make this trowel suitable for hard or stony grounds.They are very resistant to usure.

The "Mark II"  are tear drop shaped and pointed. These trowels are very reliable on the excavation. Their digging surface is more important. For beginners, the rounded edges avoid damage to the archaeological layers unlike the "Mark I" and "Mark III" who have sharp edges. On the average, the "Mark II" are thicker than the "Mark I" and they are also suitable for stony areas.

The "Mark III" are triangular shaped and offer the highest digging surface compared to other types. They are very reliable with sharp edges allowing you to dig hard grounds. Like the "Mark II", the "Mark III" are thicker than the "Mark I".

The blade-length : 
For an adapted use in archaeology, the blade-length of a trowel should be between 7 cm and 14 cm. A length under 7 is often not adapted, you would prefer the use of a spatula. With a length over 14 cm, you will work too hard with a trowel. In this case you better should employ a hand pick.

The thickness of the blade depends on the soil  to dig :
A thin blade is "sonorous". For loamy-sandy humid soils the vibrations transmitted by the blade will indicate you changes between the different layers. But for stony areas, a thin blade will wear out more rapidly. A thick blade is "silent", no vibrations transmitted, so you will find it difficult to feel the layers. On the other hand, the thickness is an advantage for the digging of stony areas.