I think you will be able to have a good look at the organization of the white and red pulps, the vessels, trabeculae and so on cross sections. You will be to follow the blood supply of the spleen from the hilus into the deep tissues and this is another advantage of cross sections and this also will reply your question about which part is better to look at, the upper or the lower part, where in similar research to mine ...following some antigens through different organs and the localization of these antigens showed variations in the localization from upper to lower parts or outer to inner parts (spleen and bursa of Fabricious) where upper or outer parts were infiltrated with more cells containing the antigen than the deep or inner parts.... so depending on the aim of the sections you will be able to decide ......I attached diagram of cross section of the spleen...hope this will help

I think cross sections are better because getting sections from upper capsule to lower capsule including all parenchymal tissues ( white and red pulps) will provide you with a clear idea about what is going on inside the spleen with the recommendation of using serial sections protocol. 

but in the case I use a longitudinal section, wouldn't it be better to have a look at the organization of the white and red puld, the vessels, trabeculae and so on?

And in the case I use a cross section, is any part of the spleen worthing to look at? Is it better the upper part? the lower part? 

I think you will be able to have a good look at the organization of the white and red pulps, the vessels, trabeculae and so on cross sections. You will be to follow the blood supply of the spleen from the hilus into the deep tissues and this is another advantage of cross sections and this also will reply your question about which part is better to look at, the upper or the lower part, where in similar research to mine ...following some antigens through different organs and the localization of these antigens showed variations in the localization from upper to lower parts or outer to inner parts (spleen and bursa of Fabricious) where upper or outer parts were infiltrated with more cells containing the antigen than the deep or inner parts.... so depending on the aim of the sections you will be able to decide ......I attached diagram of cross section of the spleen...hope this will help

Thank you very much! That's supernice! :)

You are welcome...Good luck