First generation have prominent CNS effects, usually drowsiness (which an be useful, eg to help sleep in chronic itch) but can be hyperactivity, paradoxically!
Second generation have lower incidence of these side effects.
Benadryl (brand name) can be either cetirizine or acrivastine. Latter marketed as “fasted allergy capsule” – I suspect the only allergy capsule.
Tmax (plasma) lowest for Rupatadine (0.75hr), levocetirizine (0.8), cetirizine (1). Chlorphenamine is 2.8, worse than diphenhydramine (best of the sedating antihistamines at 1.7). Loratadine (1.2) and Acrivastine (1.4) not far behind; Desloratadine has a range of values from 1-3, which is unhelpful, Fexo is definitely slow at 2.6.
In terms of clinical effect on wheal (because tissue distribution different from plasma), cetirizine, acrivastine, levocetirizine equivalent (1hr), diphenhydramine, fexo, desloratadin 2 hrs, CPM 3hrs (!). Rupatadine is 2 hrs, despite the short Tmax!
No particular reason to think formulation matters. Possibly liquid might work directly in the mouth and throat, but only if you don’t swallow it immediately (gargle!?).
Some examples of capsules beating liquids for specific drugs (!?) but prob not much difference between absorption from tablet, liquid etc. Prob easier to swallow liquid if oral angioedema, but liquid bolus might actually be harder than tiny tablet…
[J Investig Allergol Clin Immunol 2006; Vol. 16, Supplement 1: 3-12]