I had the Minolta 75-300 (and the Sony is optically the same, I
believe).  It was sharp and contrasty through about 200mm, after that a
bit on the soft side.

Sold it long ago.

As all this is new to you, be advised that the rule of thumb for sharp
zooms is a zoom ratio not greater than about 2.5:1.  The lens you're
considering is 4:1.

Consider 2 lenses rather than 1.

Cheers,
Alan

If you are new to SLR photography you will get more use and enjoyment out of
the 18-200 then you will out of lugging around a second lens in the 70-200
range when you want to have telephoto capability.
Unless you buy top end glass there is not a lot to distinguish the lenses
from different manufacturers in the middle of the price range.
I do not know about the Sony 18-200, which I presume is a new lens for that
mount.
I have used a variety of the 18-200 lenses out there for dSLRs and for
general use these are very good lenses, much better than the old 28-200
lenses for 35mm SLRs and equal to or much better than most moderate cost
70-200 zooms in that focal length range. At the wide end they have more
barrel distortion (easily correctable in Photoshop) than your kit lens,
which you should use unless you need the long end of the zoom.
If you learn how to use Photoshop and do not plan to routinely make16 x20
inch prints the dirty little truth is that absolute lens quality does not
mean what it did in a world of film based images that received more or less
straight printing from a negative or transparency.
In a digital world where all images receive software sharpening how does one
know what one is seeing in a print?

It all depends on your use...  I have the Sony 18-200 which I use as an all
purpose 'walking around' lens, one advantage is I never have to change lenses so
I avoid dust... I carry this camera everywhere to take snapshots...   If I
wanted to take more 'serious' photos I would use a prime lens, (no zoom) but I
have a Nikon to do that with. I have a 24mm F1.8, and a 50mm F1.8. I don't have
a prime telephoto since I haven't decided on which one yet... but around 200 or
300 would be good. Tripods are required for a larger lens.

Ask yourself what you want to shoot, first... and what quality you want.

Quite a few Konica Minolta 75-300mm (D) lenses have been around from
prices around £49.50 upwards, new, remaindered stock. They may all have
gone but getting one for around £99 as opposed to the £150+ you are
likely to pay for the Sony version (very slightly improved in terms of
feel and probably better quality control overall) would be worth the
difference.

To go with your 18-70mm, this is a good choice. It is light, relatively
small and really adds fresh tele range with no overlap. It shares the
same 55m filter thread, for UV or polarisers. Overall peformance is
surprisingly good for a cheap lens, but it's best to stop down one stop
- f6.3 to f 8 is a sensible working setting.

The Sigma - the real question here is 'which Sigma?'. The cheapest Sigma
is not likely to outperform the Song/Konica Minolta, while the more
expensive APO labelled version(s) will probably be better. Sony has
temporarily discontinued the former Minolta/Konica Minolta 100-300mm
f4.5-5.6 APO (D) lens, which is or was the best possible choice for
digital bodies - more compact still and very sharp, but twice the price.
They may be reintroducing it, everyone hopes with a faster focus
mechanism or even perhaps an ultrasonic focus motor. This is the only
reason anyone can think of as to why it has been dropped, and only the
cheap 75-300mm remains in the line-up as a basic tele zoom.

So my answer would be - if you can afford a Sigma APO variant, get it;
if not, save your money, find a cheap final stock or used Minolta/KonMin
75-300mm (D) and survive with that until Sony introduce further lenses -
or begins to drop the price of the initial models.

David