Agree with Skin on this one. The vdrum is gonna be noisy, the
alesis is not user friendly. So the Simmonds wins in this scenario. Agreed too the
yamaha DT series is worth saving a little more.
however...
You would like to be satisfied for a long time. any of these kits will not satisfy you for a long time.
Your Electronic drum requirements do you plan to play live, out of your unit where you need to plug in to a pa and have some durability? if so then consider this.
The brain is probably the most important aspect. As are the other components. This is the element you will want to have good upgradabilty on.
Without doing tons of research, I would assume the $500 range is far from upgrade able. ( prove me wrong)
how about building you own electric kit, that is good quality in the price range you describe and fully upgrade able in every detail as you collect more dollars.
As a starting point
http://members.cox.net/ampage/triggers.htm
Then find a brain system that you like.
My considerations would be mono/ stereo samples output. Is a snare drum actually in stereo when you hit it? Well yes if it is stereo miked. how often does anyone stereo mike a snare drum for live application? If you send stereo signals from a brain to desk what sort of phase cancellation is likely to occur? ( sorry getting technical)
Next sample frequency. The
alesis boasts 48khz woo hoo. This would be a minimum.
The samples themselves. Can you use any sample, upload it after a file conversion and tweak the parameters?
Hardware, the Vdrum ebay one looks kinda cool but also really lightweight and you would find it bouncing all over the shop.
Can you use after market pedals? spare parts like lugs, screws etc.
and the brain, is it user friendly, has program safe, search and save? can you adjust individual channel parameters and does it have XLR or TRS outputs. 2 headphone outs.
My guess is for a beginner who has never played drums or used midi gear your excitement factor will last maybe 12 months then you will outgrow it.
If you can already play, have an understanding of 'brains' and samples, 3 months and it will be on ebay.
So while this is long winded in short the Simmonds is the better deal.