Hi David:
The question was originally about the tail vise, but many other excellent
points have been raised,  so this is now really just a...
General Bench Discussion

I am currently using my 4th  bench, so I can honestly say I have tried
pretty much everything for extended periods of time --ie for years, not
just a couple of weeks.

Bench style: As someone stated earlier, it really depends on what you tend
to make and do with your bench.  There really isn't a one size fits all
solution to this.  It all boils down to the size of the stuff you tend to
do and how you work.  If you are making huge mortise/tenon entry doors all
the time vs someone who makes  desktop humidors, the needs are very
different.

 Workbenches have to be heavy (immobile), reasonably flat, and strong, and
hold the work safely and securely.

For me, a large quick release Record front vise is perfect- it holds the
size of stuff I typically work on solidly (6 feet long or less).
.
Holdfasts are also very handy, but tend to eventually fail over the years
if you use them alot (the holes gradually enlarge, and then starts the
slippage).  I prefer vises to clamps, but whatever floats your boat, eh?

Tool Tray- My third bench did not get a tool tray...  Unfortunately I got
sucked into what woodworking guru's were saying about tool trays and bench
heights, so I made that third bench way too short and without a tool tray!
    So tool trays are a must have some people, but you may be different. So
pay no mind to the famous guys who claim they're for hamster nests- know
yourself and make it the way you  actually work.   If you make a chisel
cut, then put on the protective cap back on, and put it back into your
chisel roll after every cut, well, the tool tray may not be for you....

A valuable lesson: guru's don't know jack shit about you or how you work.
Know thyself, and build accordingly.

So yes, you should consider both a front and a tail vise, but, depending on
what you typically make, you could conceivably do without either (with
appropriate design considerations).

  A tail vise is a pain to install after the fact, so I would definitely
advise you to plan carefully on that front.  Front and leg vises don't need
too much roughing in, so it is easy to design your bench such that you can
fit them in easily without compromise if you decide you want to.

Cheers from Waterloo
Claudio