I am a native Croatian speaker and I want to tell you that the Serbian Cyrillic alphabet shouldn't discourage you from learning Serbian if that is the language you want to learn. I learned a couple of Cyrillic letter to be able to read Serbian texts and for me it wasn't so hard. You just need some time to get used to it.

If you are, however, still terrified from the Cyrillic script, then just use transliteration for Latin <> Cyrillic conversion. Each and every letter of the Latin alphabet can easily be converted to Cyrillic and vice versa. The spoken language doesn't depend on which script you are using, it is the same. I'm quite sure that all Serbs can read the Latin alphabet (it also gets used online on forums), so the alphabet shouldn't be an excuse.

Regarding what is easier, since the grammar is practically the same, I'd say Serbian by 1%, because Croatian has: "ije"/"je", whereas Serbian just "e".

Left: lijevo / levo

Milk: mlijeko / mleko

Where: gdje / gde

I would like to point out that when you speak, you don't hear "ije" or "je", but "je" with a longer or a shorter accent. This length of an accent is the same as in Serbian, so the difference is minute, only in the written language.

Final verdict: learn the language you will actually be using more. If you feel you'd be using Croatian more, then learn Croatian.

Start the one you could get the study books for. They are almost the same.

:-)

You should probably read this blog-article which is written by italki professional teacher Magdalena:

http://speakupserbian.blogspot.co.at/2014/09/how-hyeck-cyan-you-understyand-yeach.html

@ernesti: I cannot follow your problem of installing a cyrillic keyboard (the letters are at the same place as they are for a croatian letters + there are 3 letters more)

I fully agree with walter_mellon- and the book is very good!

There is so little difference between Bosnian, Croatian, and Serbian that the leading textbook teaches them all simultaneously. Why not learn all three? A small extra effort would allow you to know each one. Serbian uses the Cyrillic alphabet; you can make flash cards and learn it in an hour. 

Get the textbook Bosnian, Croatian, Serbian, a Textbook: With Exercises and Basic Grammar by Ronelle Alexander.

I think it depends more on what you want to achieve. If your main concern is to be able to understand, be able to listen or read, than it is not going to matter. I would even argue that in that case you really don't have to choose. If, on the other hand, you want to be able to speak correctly, do yourself a favour, choose one and stick to it. They are mutually intelligeble, but they are not the same. Plus, there are not only differences in vocabulary, but also in grammar (often very nuanced). In my experience, those books that deal with "Serbo-Croatian" usually focus on Serbian or, more confusingly, try to cover as many differences as possible. This must be very taxing on the learner. You really don't want that. Learning a Slavic language is already a challenge enough.