Some botanists treat O. dillenii as a variety of O. stricta, and some treatments don’t even recognize it. Both plants may grow in similar habitats, but they are dissimilar. One important difference is that O. dillenii generally has pads with scalloped edges, whereas O. stricta does not or has less pronounced edge crenations when such occur (rarely). Additionally, O. dillenii is a much spinier Opuntia than O. stricta.
Danny Green reports:
The cladodes on the O. stricta are generally oval or spatulate, whereas those of O. dillenii are obovate and/or rhomboid.
The areoles on O. stricta tend to be flush with the surface of the cladode, whereas the areoles on Opuntia dillenii are elevated above the surface of the cladode. For this reason, the margins of the cladodes on Opuntia dillenii are scalloped or wavy unlike the to be smooth or straight margin of O. stricta.
The leaves of O. stricta are shorter, thicker, and pointed upward (more parallel to surface of the cladode). In contrast, the leaves of the other Opuntia tend to be longer (though not all the time), slimmer, and pointed outwards (more perpendicular to the surface of the cladode).
The spines on O. stricta tend to be straight and perpendicular to the surface of the cladode, and it is common for them to be mottled with brown and yellow. In contrast, the spines on O. dillenii tend to be slightly curved (sometimes very noticeably curved) and usually yellow without mottling. Populations of O. dillenii in the Florida Keys may have mottled spines. John Kunkel Small thought that these Keys prickly pears were a separate and distinct species and referred to it as O. zebrina.
O. stricta tends to be more shrubby in habit or even prostrate-ascending, usually between 2- to 5-ft tall. Exceptional specimens of O. dillenii may be 10-ft tall and have a very discernible trunk. However, it is more common to see shorter O. dillenii plants that are 3- to 6-ft tall. Irrespective of height, O. dillenii is seldom prostrate-ascending.
Finally, cotyledon morphology separates the two Opuntia species. The cotyledons on O. stricta are heart shaped, half as long as those of O. dillenii, and pointed outward, whereas the cotyledons of O. dillenii are oval, elongate, and pointed upwards.