Hello there,
I am a long time Fender fan. I used a Bandmaster for years and years. Just wanted to state my bias.
If I had to choose between only those 2 combo amps, I would go with the Line 6. Easy choice. As much as I love Fender, I would take the Line 6 Spider 75 watt 1x12 combo over the G-Dec 15 watt because the Line 6 has a better speaker. I am not a fan of the small practice amps (10 to 20 watt jobs), no matter what brand. I think they have lousy speakers that make you sound thin and tinny. How the amp sounds is more important than what built in effects it has.
One thing you should know about the Line 6, is you may not be able to reach all the advertised presets without the right footswitch. I have not played a Spider iv yet, so I do not know if that changed from the earlier versions. I do have a Spider iii 150 watt 2x12 combo. For that you need the FBV shortboard. The FBV-2, FBV-4 and Express pedal only lets you switch between the 4 basic channels. To access all the rest of the presets, you need a shortboard to get to them. I have picked up a shortboard and scrolled through the myriad of presets. Fun for a day or so. Now I just use the FBV-4 pedal to switch channels. I can set those 4 to what I really want. I doubt if I will ever use all the rest. However, I can tell you this, the speaker in the 75 watt 1x12 is the same model speaker has what I have in my 2x12. It is a decent sounding speaker. True, you won't need 75 watts of power in your bedroom. No big deal. Keep the master at 2 or so. At least you are not going to run out of headroom with that amp.
You can down load whatever backing tracks you want. There are many available on-line. Just play them from whatever you want or run them through your amp. That is not a big deal. Besides, backing tracks don't help you learn to play, they don't make you play better. What matters more than the backing tracks is the speaker in the amp.
Oh, by the way, I started playing in 1957, so I am pretty old school in my thinking. In addition to my Fender amps, I have a Vox, a vintage Marshall Lead, a B-52 head and a Marshall 1960 4x12 speaker cab sitting here by the Line 6.
Later,