SLL is very similar to CLL, morphologically, phenotypically, and genotypically identical, infact they are often referred to as CLL/SLL. The only difference is in lymphocytic count <4000 cell / mm3 is SLL but most commonly that is not the case as SLL represents 4% of all NHL, while CLL the is most common leukemia of adults in the western world. Asyptomatic patients are very common, others have painless lymphadenopathy and spleenomegaly (60%), these sites + bone marrow and liver are involved histologically in almost all cases, they have a very benign course. They are formed of sheets of densely stained small nuclei with little cytoplasm, smudge cells are common (cells are fragile and break down during slide preperation). There maybe variable but small number of large cells. Cells express CD5 (+Mantle cell lymphoma) + Bcell CDs.
Diffuse large cell lymphoma is one of the most common NHL in adults (FA says follicular lymphoma is the most common), may occur in children, maybe derived from T cells (20% and 20% respectively ). They are composed of large cells with very large vaculated nuclei with multiple nucleuli and dispersed chromatin. They may have t(14;18) translocation involving the BCL2 gene (these are originally follicular types (30%)). May occur anywhere but commonly in GI and brain, while spleen and liver are not common sites early. In immunocompramised (AIDS and transplantation) EBV is implicated. They are very aggressive and rapidly fatal if not treated.
The only thing that's common between these two is that they occur in the elderly and they are Bcell NHLs.