My EF-S 18-135mm for a lens with more zoom for the EOS 50D.

[foram456teen]

I’m considering swapping my Canon EF-S 18-135mm for a lens with more zoom for my EOS 50D. I’ve read recommendations for the Canon EF-S 18-200mm f/3.5-5.6 IS and the Sigma 18-250mm f/3.5-6.3 DC. Could you highlight any key differences between these two lenses? Are there any standout features that make one better than the other? Or do you have any other recommendations?

[deebatman316]

The EF-S 18-200mm F/3.5-5.6 IS lens uses a very old and slow AF motor called Micro Motor. The newer versions of the EF-S 18-135mm F/3.5-5.6 IS lens lineup use very fast AF motors those are STM and Nano USM. The original version used Micro Motor which has been retired by in 2012. Micro Motor was first released in 1993 and is very slow compared to newer lenses. I wouldn't recommend lenses with a zoom ratio of more than 1:4. The EF-S 18-200mm F/3.5-5.6 IS lens has a zoom ratio of 1/11. The Sigma 18-250mm F/3.5-6.3 DC lens has a 1/14 zoom ratio. Also the small aperture at the telephoto may cause AF system problems. No Canon EF or EF-S lens has an aperture smaller than F/1.4 only 3rd Party lenses do.

[Tronhard]

Welcome to the forum!
If you can afford to I would retain the excellent 18-135 lens you already have. 
Also, if you have a budget for this then the $ value of that would extremely important to know.

The focal range you need will vary with the kinds of subjects you are going to shoot, so some advice on that would be helpful.
However, using what you have suggested either a Sigma (canon mount) 100-400 or Canon EF 70-300MkII IS USM would be suitable.   You could look on used and refurb sites KEH.com in the USA to save you funds, they come with a warranty and have a good reputation.  Avoid on-line 'bargains', they usually are not.

[Tintype_18]

Good advice on the various lenses. I would keep the 18-135 and complement it with another as suggested.

I have sold two items in the past and regretted it for years. To replace them is 4-5X the original cost.

[ebiggs1]

IMHO, the gain from 135mm to 200mm isn't that much. If you want a real difference keep your current lens and add one like the 100-400mm.