I played a Susato for the first and last time at the Northeast whistle gathering…
Robin
P.S. Who the heck is the whacko with the bell? He looks vaguely familiar.
P.P.S. You might like to try a Dixon tunable. Not too expensive.
Well in my ahem infinite experiance with whistles points to little avatar pic thing which shows every whistle she’s owned so far, I would say an Acorn is a good buy in terms of range and out-of-the-box playability. I really didn’t have to do much work at all to it (in comparison with my Gen., An Seisiun, and even Clarke). I’ve also been able to get higher notes out of it than I have any of my others, yet the lower octave is still nice. shrug if you’re looking into one of the cheapies.
unless it’s only to play with your guitar friends (that’ll teach them, those guitar players!).
…I’ve been playing guitar for about 10 years now, and it’s become a love and a passion for me. I also play the tin whistle as well. I’d be glad to teach YOU some things… 
Anyway, since most tin whistles are tuned to the key of D, then perhaps the reason everything was sounding out of tune was because maybe your guitar-playing friends did not realize that they have to play in the key of D in order for everything to sound harmonious. Either that or they just don’t know how to tune their guitars in the first place.