Electroporation vs gene guns
I am not an expert on gene guns but I will try to give my answer on this question anyway.

Price-wise they are both relatively cheap. The electroporators consumables are only the cuvettes, they cost 5-10 dollar/each. The gene guns use microbeads as bullets and depending on the material they cost between 200 and 300 dollars per microgram, 10-50 dollars per shot, a bit more expensive than electroporation.

About the organisms, guns are used mostly for plant-cells and few other "hard to electroporate" organisms. I don'd have a proper list of them but roughly, if you work with bacteria, yeasts and mammalian cells, then go for an electroporator.
If you need to work with tissues or entire organisms like worm/insects/plants then go for a gun, you definitely cannot electroporate a leaf. But in general, check the literature on what is currently the best method to transfect your system of choice.

Efficacy, also in this case it depends form the cells you are working with. Electroporation reach 10^9/10^10 transform ants per microgram of DNA in case of E.coli and this is the best efficiency you can get nowadays. All other cells will be transfected at lower efficiency, how much lower? It depends from the cells!

Easy of use. They are both very easy to use, it is matter of pressing a button. For the electroporation you need to prepare electrocompetent cells, but the procedure is very easy. For the gene guns you need to coat the beads with DNA, also very easy.

Maintenance. The electroporator do not need any maintenance at all while the gun may need some care in the long run since it has pumps and pressurised chambers, this stuff usually need some attention but nothing special.

Some references:

bio-rad.com/LifeScience/pdf/Bulletin_9541.pdf

bio-rad.com/webroot/web/pdf/lsr/literature/4006174B.pdf

bio-rad.com/webroot/web/pdf/lsr/literature/4006217A.pdf