Grovers !!!
I have two Grovers, one is 21 1/4" and the other is 24 1/4", both with 6" bells. It comes down to amatter of personal preference but I couldn't be happier with what I have. Ricksons Wheels is a great source for the horns, give them a call @ 1-800-587-7663 and ask them to send out their horn horn cataloge. They have alot to choose from.
I can't respond to the question on the electric horns as I have never heard them, but IMHO, you will probably be more satisfied with actual air horns over the electric ones, especially if you are looking for a deep tone. If money is real tight, purchase one of the horns with the larger bells now, you could always add another at a later time.
You don't have to spend a bundle either. You can pick up a nice set of black powder coated horns for around $50. 00 and couple of $$$ for the solenoid valve. You also don't NEED the compressor, a aux. tank aired to 125 psi's will last awhile, depending on it's size. Most everything you need, sans the horns and the solenoid valve, can be purchased at a hardware store for a minimum of $$$.
I'm not using a compressor yet, my source is two aux. tanks piggybacked together and concealed in the toolbox. Works great.
If you want to hear mine, send me a PM and I'll give you my home #. Give me a call tomorrow and I'll place the phone in front of the truck and let 'em rip for ya

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Scott W.