I'd also go with the 10" slider and a compound one at that.
I recently did all the interior woodwork on one for my house (trim work, window moldings, stairs, door frames, baseboard, etc) with both a radial arm and and 10" compound slider. The slider was very useful and beyond something that was really wide (some of the 12"+ trim boards required a spin) it was the "go to". I setup the radial arm on a perfect 90 and did everything else on the slider.
On my slider you can also lock the slide portion so it becomes a non-slider if one needed too.
Depends on the type of work you'll be doing.
Most sliders I've seen have too much slop for fine trim work. I'm sure there are better available if you have the $$$.
I have a cheap 10" slider I use for framing and other rough work.
For finer work I have a mid-level 12" non-slider.
Sounds like something is wrong. I've used a number of different models ones and never experienced any slop unless it was one of those really inexpensive units.
With a good blade on mine it cuts just as smooth (if not smoother because I can control the angle of approach into the work) as my radial arm or non-slider.