Typically, iNet refers to a mobile-data (only) centric plan, say for a 3G tablet, while an iSMart package refers to a combination of voice minutes and data (or a lower per minute voice rate), say for a smartphone.
But you'd have to grind through the T's & C's to figure out the exact difference. Obviously in that link I'd go with the iNet package, but you can get 3 GB (1.5 GB x 2) for 399 on a double Net promo) so I'd probably go for that.
Each provider has so many plans/promotions on-going, and honors old ones forever, so it is impossible to stay up to date on all the offers.
I see TrueMove H also has double Net promos:
http://truemoveh.truecorp.co.th/topp...nlimit_en.html