Welcome aboard MeliCunn!
May I first tell you that no matter what choice you make for your "first time" Im 100% you`ll have the time of your life!!! First trips of this kind are ALWAYS amazing experiences! You cannot go wrong!! Follow your heart and thats what there is to it!
Experiences may differ but considering Sarria to Santiago there are two points of view (at least for me):
1) Sarria to Santiago is a Beautiful walk! There isnt a lot of strugle and not very tought streatches at all ; Galicia is simply beautiful! Wonderful scenarios, nice people along the way.... And ... well. .. due to the "I-MUST-get-a-piece-of-paper-called-compostela-and-place-it-on-my-wall-at-home" 100 Kms to Santiago Shenannigans, it ends up being the most popular part of the Camino. And Indeed, I would say MOST first timers walk that stretch specially because of the Compostela...
2) Sarria to Santiago IS the most popular stretch of the Camino, and for that long, it ends up being one of the worst parts for those who are walking longer stretch, or walking the "whole" thing (There isnt such a thing as the "whole thing" anyways...). There are thousands of people with their shinning new gear making all places look like a circus. A lot of groups walking together, a lot of booked places, full albergues, and people racing each other along the way....THe weather in GAlicia as many already knows can be very very Wet! THat can be a hard experience for first timers leaving home and being wet the whole trip!
Crowd conditions may improve once the 300km limit is established, if this is done at all...
I know one thing for sure: All people Ive met that have walked this stretch as a first time, ended up loving it the same way! But, I`ve always have the feeling that if I had to walk only this part I`d be frustrated and disappointed.
That said, I have to agree with both John Finn and Kinky One.
Both are right on their statement....
My personal opinion: I`d do some further research and consider other stretches of the FC before making up my mind but, as I said in the beggining I think you cant go wrong on walking any part of the Camino as a first time!
Buen Camino!!!