Thing is, the jigsaw doesn't come into it at all because it will run of 15v and 18v so which ever route you take the jigsaw will always be your friend.
Same with sys lite he will always be you'r friend which ever voltage you go for.
The only tool which would decide which route you would have to take is the Ti15
I can and can't see festool bringing out a Ti18
I CAN'T because as you all are fully aware of my opinions of the Ti15! I can't see festool have made alot of sales in the UK with the Ti15 so would seem silly to spend/invest in a Ti18 unless it was improved.
I CAN see festool bringing out a Ti18 because wouldn't be right not to, seen as they have started the 18v range now and people won't be happy if festool starts to just have certain tools in certain voltages unless they are like the jigsaw and can work of any voltage.
The reason why most people talk about the T15 more than the T18 is because
1. Cheapos some didnt want to spend that little extra!!
2. Some people might want the Ti15
3. Like me some might of bought the T15 when the T18 wasn't even available.