jtoomimnet requires more CPU and less RAM than mainnet ("master") p2pool. Because of this, it is strongly recommended to use pypy with jtoomimnet, as pypy trades off higher RAM usage for lower CPU usage. If you use pypy on jtoomimnet, you should have lower rates of invalid (DOA+stale) shares than when you mine on mainnet.

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Yes, jtoomimnet has more hashrate. This is unlikely to change. jtoomimnet makes larger blocks and collects more fee revenue. If p2pool finds a block within about 30 seconds after the previous block, then jtoomimnet will make a ~999 kB block (plus whatever Segwit bonus there is) but mainnet would make a ~100 kB block. After 60 seconds, it's 999 vs 200 kB. After about 300 seconds, they reach parity and mainnet can mine full blocks. If you wish to verify this for yourself, wait for a block to be published (e.g. check blockchain.info), and then check the "Current block value:" on these two nodes:

http://crypto.mine.nu:9332/static/classic/ (mainnet)
http://crypto.mine.nu:9330/static/classic/ (jtoomimnet)

You can also use the share explorer to look at how the share/block values change over time. For example, this is the first share mined by p2pool on top of block 482381 for each of the two networks (links will go invalid the next time sawa restarts his nodes):

http://crypto.mine.nu:9332/static/classic/share.html#0000000000000e27a506eb2eacacdb12ae77cd1c28fe23789e5da35edd001d69 mainnet 12.66496688 BTC
http://crypto.mine.nu:9330/static/classic/share.html#0000000000000df957e19f5712d4efbf04bc825ab673a1e0db2e696bd8ce9961 jtoomimnet 15.53978325 BTC

Those two shares were mined within 1 second of each other. If the hash had been good enough to be a valid block, then jtoomimnet would have earned 2.874 more BTC (22.7% more) than mainnet. Here's a quick table of the differences for the first few shares after block 482381:

Share#	BTC_difference	jtoomimnet_btc	mainnet_btc
1	2.87	15.54	12.66
2	2.69	15.54	12.85
3	2.50	15.57	13.07
4	2.82	15.69	12.86
5	2.79	15.76	12.97
6	2.36	15.81	13.45
7	1.95	15.97	14.03
8	1.78	16.00	14.23
9	3.53	16.07	12.54
10	1.69	16.13	14.44
11	1.34	15.98	14.64
12	1.48	16.14	14.66

The payout per block will sometimes be the same between the two networks, especially if the current block round has been going on for a while, but when there's a difference, it should be big and in jtoomimnet's favor.

The cost of the change that allows higher revenue on jtoomimnet is that p2pool has to deal with more transactions in the share chain. This increases the CPU, RAM, and bandwidth requirements for jtoomimnet. I added optimizations to the code that more than offset the RAM requirements and partially offset the CPU requirements. I have not yet addressed the network requirements, nor have I finished addressing the CPU requirements. Unfortunately, I'm a lazy loafer and only spend on average a few hours a week working on p2pool code, so things don't get done as fast as I'd like.

If you have a fast enough CPU and networking, and enough RAM, and if you can get pypy working, then jtoomimnet will have better expected revenue (bigger blocks), more frequent payouts (higher hashrate), fairer payouts (fewer orphan + DOA shares, smaller advantage to large miners), and will be better for Bitcoin (larger blocks) than mainnet (v17). jtoomimnet's support for Segwit also appears to be less glitchy than mainnet at the moment.

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I expect I will have the code ready to have jtoomimnet fork into a Segwit2x and a Core chain before the Bitcoin fork happens. I would prefer it if nobody mined on the Core chain, but I think that freedom of choice is more important than having people follow my choice. The code for auto-forking is about 40% done so far.

The jtoomimnet code has some problems with altcoins still, and I haven't had a chance to investigate/fix them yet, so jtoomimnet is not ready for p2pool master. I would also like to fix the CPU usage (and maybe the RAM and network usage too), but whether I do that before or after merging into p2pool master is TBD.