Magnification is calculated as scope focal length / eyepiece focal length (so shorter focal length eyepieces give you a more magnified result).
And the more you magnify, the more the captured light is spread out, so the image gets darker the more you magnify it.
Also - and more importantly - you eventually get to a point where magnifying the image more doesn't show you any more detail, it just gets bigger, dimmer, and fuzzier.
Theoretical maximum magnification is detemined by aperture - the size of the objective lens (for refractors) or primary mirror (for reflectors). The bigger the scope, the higher the maximum useful magnification. For everyday conditions, the maximum useful magnification is around the scopes aperture in mm (and you get it with an eyepiece with focal around the focal ratio of the scope - so around 6mm for an f/6 scope.
Given good seeing conditions (steady atmosphere) you can go to around double that magnification, which is closer to what what manufacturers usually quote. What they fail to say is that seeing conditions good enough to allow it can be rare.
However, other factors also come into play. Inexpensive refractors are usually what's known as "achromatic" - they don't focus all colours of light at the same point, which causes what's called "chromatic aberration". This usually shows up as a violet halo around bright objects, which makes the high magnification performance suffer (slower long tube refractors - with larger f-ratio numbers are less affected than fast, short tube refractors). By contrast, reflectors reflect all colours of light the same, so don't suffer from this.
You can get more expensive refractors - ED or Apochromatic ("Apo") designs that have much better colour correction than achromats, but the cost goes up significantly.
However, don't get too hung up on magnification - it's handy for looking at planets or the moon, but many deep sky objects actually look better at lower magnification; a bigger scope helps here because it captures more light.
Reflectors have the advantage when you're looking for a large light bucket; you can get reflectors with much bigger apertures (8 to 16" or larger) than you can with practical amateur refractors (around 6" or so).
On the other hand, refractors don't have a central obstruction, and tend to give higher contrast views than an equivalent sized reflector - but reflectors don't suffer from chromatic aberration (but fast reflectors suffer from coma, which refractors don't).
Note that newtonian reflectors aren't really suited to non-astronomical uses, while refractors usually do better for terrestrial use (depending on the diagonal, you may end up with a left/right reversed view, but at least it won't be upside down).
With entry level scopes, you'll probably find that a dobsonian style mount (as used by many reflectors) is more stable than the tripod for a refractor; entry level scopes are built down to a price point, and for scopes with a tripod, that usually means that the tripod is fairly flimsy and vibration prone - so it may take a little while to settle down after adjusting something (either pointing or focus).
Of the two scopes you mention, the refractor is likely to do better at low power wide field views than at high magnifications. With no chromatic aberration, the reflector is likely to do a better job on planets.
