I've probably done more "serious" playing on an old, cheap Eb tuba than anyone else on this board. That said, absolutely, completely, none-of-the-above!
If you already have a decent sized Bb tuba then a large vintage Eb tuba (50/50 chance or worse of having "unusual" intonation) is the last thing you want. Look for a small King or Conn Eb. The default intonation will probably be much better and you'll have a bigger contrast to your Bb. Try to find a front valve horn so you can yank a few slides if/when necessary (when=at least for 123 combos; tuba players that don't pull for 123 are evil
).
I bought a very nice original 4 front valve 1916 Conn Giant Eb last fall- great sound, incredibly easy to play, bad at first but eventually learnable low register,.... Everything was fine to great except the intonation. I managed to play it in a decent college wind ensemble for a while, but after that I just couldn't handle it anymore- flat 2nd partial Eb and D, sharp 4th partial Eb and D, flat (but almost learnable) 5th partial,.... I really did like the horn and almost regret not keeping it and putting a main slide rod on it but back then it was just too much for me. These days I play my little pre-1915 King/H.N. White Eb tuba (profile picture) or my Kanstul contrabass trombone on low brass parts.
Summary- buy a small vintage Eb tuba, a Yamaha 321, or a big Bb euphonium instead.
I played one of these in worse shape for a few years as my only tuba, and I actually played better and more comfortable on it than on my mildly modded King Eb-
http://cgi.ebay.com/Antique-C-G-Conn-TU ... 2307976b51
5th partial tended flat but was easily learnable. The only slide pulling I did was for 123 combos (I tuned 3 for 13 and very, very rarely had to push in for 23) and occasionally the
6th partial. I played a Bach 18 almost exclusively in mine- it stuck out a bit but was doable. A small shank F tuba mp would be a good start. I sometimes use one of several 30mm mps in my King (1 old Eb tuba mp, 2 different contrabass trombone mps). I made a lot of money on it, had a huge amount of fun with it, and only sold it as I was trying to get a job playing trombone on a cruise shop and figured I'd buy a newer bass tuba when I got back. Turns out ol' Bob can't play tenor trombone that well, and it got worse when they kept giving me harder parts (first time was backup bass parts, second was small big band parts, 3rd/last was solo combo parts
)...
FWIW, every now and then I would amaze myself and the people I played with with what I could put out of it. I played a pedal C in a "reverse pyramid" thing at the end of a song as the only tuba in a large community band and had other people in the band afterwards tell me they felt the floor rumble.
Another time I played a pedal C at the end of a brass band piece that went mega-phonic over the whole band, 2 choirs, and an organ! You do not need a big tuba to produce a huge sound- focused air will go a long way. If it weren't for that bell, I'd be severely tempted myself!