Hiya, over the years i've experimented widely with different homemade fat/suet/lard recipes and mixtures, as well as different bought ones.
In a nutshell, i'd say that the best (and prob cheapest?) option is to make your own from suet (not lard).
Re: what's best to put in them - i've spent a lot of time watching birds eat these things, and what they're after is the fat itself, not the seed inside, which makes sense, as fat has a lot more calories than an equal weight of seed. So if you watch closely, the tits, starlings, blackcaps and Myrtle Warblers etc are gobbling the suet, and allowing the seed to fall to the ground for the chaffinches. Of course, if finches have access to the mixture, they'll tend to pick the seeds out, as they don't really eat suet. But it's better to provide this separately to avoid them having to deal with the suet. So the best stuff to put in a "titbell" (as Tony Soper used to call them) - is more suet.
Wish i had a feeding station at the moment!
Cheers.