I wanted to chime in, even though this thread is semi-dormant right now. I am a new mandolin player, having spent about a month with the instrument and the training books. It's fun. For me, ukes are primarily about chords, whether strummed, arpeggio, or with some minor picking within them. It is a very personal instrument, beautiful, but not suited to playing in an ensemble. Great for backup as I sing. Mandolin is just the opposite, more of a melody instrument, able to provide chords if needed, but really shines on melody lines and providing rhythm in ensembles. It is harder to play, requires some precision. But just as I can sit and play for myself with the uke, I can also enjoy doing the same with the mandolin. I'm not particularly good on either, but i can play for me.
The mandolin is forcing me to learn more about scales and theory and how they relate to different forms of music. I could have learned that on ukulele, but I could have learned it on guitar too.
Having said that, I think the tuning in 5ths is a lot more instinctive for me than the tuning used in guitars and ukes. I find it much easier to pick up a melody line first try than on other instruments.
I guess my real message is to buy whatever you can budget for. I bought a $50 uke to see if I was interested. Using sandpaper and some info from other websites, I was able to lower the action to a usable height and to get the intonation dead on (remarkably so). Yes, I have played $500-1200 instruments in the stores and they play really nicely, but I didn't want to invest that to see if I would be interested. Now that I know I am interested, I can consider upgrading, although I plan to continue with this one for a while (maybe until a birthday or something
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There is some great music in the world, including Irish, Scotch, Bluegrass, classical, Italian... many of these sound great on mandolin. Admittedly, some are also pretty interesting on the ukulele as well, but it sounds "right" on the right instrument.
Mandolin might be a good bridge to playing fiddle someday too if that interests you at all.
Mandolin Cafe is a great website with tons of info. Lots of performers there with instruments that cost as much as my fishing boat, but most are helpful.
I guess my thought (with respect to other posters) is that if you don't have $500 to determine your level of interest, get what you can. There are many under $150. I will agree that the scrolled F styles are probably not a good investment at the low price point, since other than looks that is just a fancy strap hangar, not really needed during the "try-out" phase. Once you decide, you can either sell out, upgrade, or keep your old one as an inexpensive wall decoration.
Take my advice or not. It is just an opinion. But don't let the entry level pricing scare you off. Buy the best you can justify, but don't let advice to spend a lot of money keep you from trying it. You can try it out cheaper if you are unsure and then upgrade later.