Worthy upgrade (from Cobra 18 WX ST II to Cobra 29 WX ST)?

BigD,
It may depend upon what your intended use is.
The model you got has a couple of nifty features that recent electronics changes have made affordable, like Dual Watch and channel scanning.
The SoundTracker "feature", though, is kind of a legacy space-waster that I believe Cobra hoped would become an industry standard, but failed to do so. It was supposed to capitalize on a style of noise suppression in voice circuits called Companding, and it's commonly used in cellular telephones and some high end FM two-way radio systems. But unless all radios in a conversation have it in their radios AND have it enabled, either they or the people they're listening to will sound distorted. Some folks take advantage of the ST switch as a convenient place to put other ...ummm.... features.
A stock, legal a.m. CB is limited to the same 40 channels and 4 watts output regardless of make or model. Some people swear every new radio needs a "tweak and peak", increasing 3.5 or 4 watts to 5 or 6 watts isn't going to get you out farther; all it's going to do is burn components out, and possibly interfere with other services (by being out-of-band). A slight increase in receiver performance might be realized by a bench tune-up, it's a tough cost vs. benefit trade-off.
The Cobra 29 series is sort of a standard against which a lot of mobile CBs have been compared over the years, and they've persisted in the inventory in one form or another for a long time. It's got a couple of things yours doesn't: a switchable Noise Blanker, for pulse style noises, like spark ignitions; an Automatic Noise Limiter, better suited for motor-type noises, like wipers and windows; and a built-in SWR meter for giving *very* general antenna condition indications.
Alternator whine is sent into CBs two ways, both through the DC line and transmitted through the space to the antenna as a very low level radio signal. It's best cured at the source, but it can be costly to shield the whole charging and DC distribution system.
The only way to reliably boost both your transmit AND receive range over what you already have is by improving the antenna. The one you got is made to be quick and convenient, and for generally short-term use. But it's a huge compromise over a "full size" CB antenna of around 106 inches. I know, hardly anyone has room for one of those, but anything less is going to result in somewhat of a loss in performance, everything else being equal.
Some folks will try to fix range problems by putting on an amplifier. Most of them only increase your output power, though, and a lot of the ones with receiver pre-amplifiers inject so much noise that they end up reducing the amount of signal you can actually hear & use.
So the easy answer to your original questions are:
No extra range from the '29, unless you've got ignition or electrical noise in your receiver that this radio's extra features can alleviate;
No new antenna, once the one you have is checked and adjusted as necessary for its location and frequency (it's made to be able to cover a very wide frequency range, but not all at the same time).
Neither one will talk farther without increasing your ERP, or Effective Radiated Power. The antenna you currently have will yield a net loss to whatever is plugged into it. The good part is that decent antennas can be really inexpensive; I've got a professional two-way antenna for "low band" that cost less than $45 with the mount.
Hope that helps.
73,
Handlebar