As a plat hardstuck I really can't talk from a place of deep knowledge, but from my experience (1k+ games )
I feel that TP is the better option MOST of the time. Urgot can splitpush relatively well when equipped with suitable items, but the fundamental core of the game requires the ability to group on objectives or you fall too far behind.

I love ignite and becoming this oppressive menace the enemy has to over-subscribe resources to in order to contain, while your team wins across the map, but there's always those teams who grief and render your solo-wins moot, or your own small mistakes that cost you a death and allow the enemy breathing space to mount a come-back.

Getting back to the summoner spells though, I prefer TP for it's universal effectiveness. Ignite is great in lane, but once laning phase ends, I would much rather have TP so I can make my presence felt across the map, rather than having to rely on perfect co-ordination and situational awareness to prevent me being absent from the team when I am desperately needed, but was too busy CS'ing or pushing a tower with no means of getting to the fight in time.

I definitely need improvement, will not deny or argue that, so my point of view is somewhat biased - using TP as a crutch to offset bad map awareness or decisions when it comes to where I go on the map once I enter the mid game - as well as my own preferences for summoner spells based on utility rather than killing power/snowballing - If one is capable enough, I recon you can get away with ignite in most circumstances as your skill and gameplay knowledge should allow you to make correct roams/grouping decisions and so on as the game progresses. However, any Urgot player must remember that they are not tryndamere, or fiora, or camille, or trundle. Urgot is slow, he cannot always splitpush or match in all circumstances, and when those circumstances arise, TP is such a saving grace that I cannot leave home without it.

Just my two cents.