I wrestled with this question last fall when I was looking to get one for DDIL for C'mas. I wasn't scrapping at the time myself so I came here for help.
The regular Sizzix machine is heavy....
really heavy! Reading that statement over & over didn't prepare me for picking the thing up in the store! Even then I didn't think too much about it...or when we wrapped it for C'mas. It hits home the first time you have to take it somewhere - or if your storage spot isn't right on the table where you'll use it.
The regular machine uses all the Sizzix dies as well as the Sizzlet dies with the converter, which is an extra $14. I decided on the regular machine so that DDIL would have more options. At the time the Sidekick was almost the same price as the regular machine if you waited & got it on sale.
I honestly prefer the regular dies to the Sizzlets for the most part. There are very few Sizzlet sets where I really want all 4 of the dies. The sets of 4 run $20 when not on sale, while the regular dies range from $9 for the small ones to $14 for some of the large ones. Both kinds are often on sale for 40% off or there's always the coupons for 40% and sometimes 50% so I've never paid full price for any of mine.
I much prefer the Sizzlet alphabets to the regular ones. They're much larger than I'd use. They're also much more expensive...and the upper & lower cases are sold separately as are the numbers & punctuation. So far I've gotten Sizzlets sets that come with upper, lower, numbers, punctuation
and shadows for each!
The only downside to the Sizzix (besides the weight & now that DDIL & I both have one we don't need to carry them around) is that they don't have Disney dies.
Oh and I forgot....supposedly you can only cut one sheet of paper with the Sidekick but I often cut a couple at a time with the regular one...at least when I remember to do it! LOL
They now sell adapters for the Sidekick that will let you use other dies with it - like from the Quikutz. I don't know if you can use both the converter
and the adapter with the regular machine. I figured for now I have enough so I don't worry about it.
I don't think you
need to get the starter kit - just check to see which ends up cheaper. You get 2 Sizzlets with it which is nice & an extra cutting pad. (reg price $3) Then they give you a few strips of paper which I don't consider important for me.
HTH
If I hadn't been planning to get that $20 roller bag at JoAnns on Saturday morning I might have succumbed! You're getting it for about the same price as a Sizzlet set! 
