The longest word in Spanish is esternocleidooccipitomastoideos, which is the plural for a neck muscle that is called the sternocleidomastoid in English.
This is quite a subjective question, and would really depend on why you want to learn either language. French is the fifth most widely spoken language (L1 + L2 speakers) with 280 million speakers, while German has only 132 million speakers. However, the two languages are spoken in different areas of the world - German is mostly spoken in Europe, since most of the speakers are German, Austrian or Swiss. Meanwhile, French is spoken more broadly, especially in African countries that used to be colonised by France. French is also the traditional language of diplomacy, although this has now largely been superceded by English. If you already speak a Romance language (Italian, Romanian, Spanish, Portuguese), French will likely be easier for you to learn, while speakers of Germanic languages (Dutch, English, Swedish, Danish, Norwegian) may find German a little easier. But it really depends on who you want to communicate with, or where you would like to visit or live.
Well, there's a big difference. For me, as a foreigner, the pronunciation and the melody seem to be the biggest difference, but I am sure native speakers would have more to say on this topic.
First, by getting a tutor and then by watching movies, tv shows and listening to podcasts. If you do this daily, or 4,5 times a day, you will progress very quickly.