Experienced players use both, along with many other fingerings and voicings. But that takes time. You'll be able to do both of those G fingering variations soon enough, and others.

My advice for now based upon my experience would be to learn to use the conventional forms first unless there's a pressing reason not to. e.g. as iterated elsewhere, the G to Gmaj7 change in "Lyin' Eyes". Most early songs you'll play won't require that. When time comes one will, you'll be further along the path in acquired tactility skill terms, but more importantly, motivated to apply the tenacity required to master them as smooth rapid accurately sounding transitions at tempo, which can be quite frustrating at first.

If you are one of the exceptional few who can also manage the alternative forms almost instinctively, by all means use them too. Eventually most do, but there's no magic 'click for instant skill' button. For most of us, if expecting to be competent with both without putting in the work, it's setting oneself up for frustration and a negative experience.

In answer to your last question, 15 months. I had played guitar for a couple of years back in 1974~75 as best I recall, starting with then equivalent of today's Yamaha C40 (nylon strung classic acoustic) and buying an Ibanez clone of a Gibson SG electric 12 months later which I played for about 12 months before a demanding dual career requiring all my attention and motivation kicked in triaging guitar out of my life.

Move along four decades flying airliners until retirement, I'd pretty much forgottten anything about guitar I ever knew. I picked up a guitar again for the first time in 43 years in Nov 2017, choosing to buy a Yamaha Pacifica 112V electric guitar. By the end of December 2017 I also had a Yamaha e-acoustic APX600 and another Pacifica, a P90'd 311H.

I started learning initially by accessing Justin Guitar's free online lessons, You Tube tutorials, by reading "Guitar for Dummies" and using "The Hal Leonard Guitar Method: Complete Edition" before recognising I really wanted affordable 24/7 accessible structured interactive quality tuition. I checked out all the usual suspects and joined Guitar Tricks in Feb 2018, so I'm about to renew here for a first anniversary.

I do have plenty of time to immerse myself in everything guitar, which is an inarguable bonus. I just wish my body and coordination was still as cooperative as my mind, a less desirable privilege of attaining age. I have since acquired "too many" guitars, amps and pedals in the opinion of my thankfully tolerant wife, and she's possibly half-right. But I can afford it and its only stuff to play with, tools to be used. I do my own setups and luthiering with the guidence of Dan Erlewhine's "Guitar Player Repair Guide" and myriad of online resources from Stew Mac to You Tube et al. Apologies for all the "I" starting sentences, but you did ask, and its difficult to rephrase a semi autobiography humbly without them even if not intending it as an exercise in ego stroking.